Mechanical Trap Minimum Cost
It doesn't look like you're pricing that trap correctly. Here's from the mechanical trap rules:
The base cost of a mechanical trap is 1,000 gp. Apply all the
modifiers from Table: Cost Modifiers for Mechanical Traps for the
various features you’ve added to the trap to get the modified base
cost.
The final cost is equal to (modified base cost × Challenge Rating) +
extra costs. The minimum cost for a mechanical trap is (CR × 100) gp.
And from the trap creation rules:
The base CR for a mechanical trap is 0. If your final CR is 0 or
lower, add features until you get a CR of 1 or higher.
By the rules, you can't create a trap below CR 1, and the minimum cost is 100g.
Trap Math
Lets look at your costs compared to the rules. The base cost is 1000g.
It is location trigger (no cost)
Correct, this is no change to the base cost.
with repair as a reset (-200gp)
Correct.
there is no bypass
No modification to cost.
the search DC shouldn't be more than 20 (no cost), if not less (-100gp
x (20-DC))
Correct, so a search DC of 15 would be a modifier of -500. Total of -700. You have no extra costs, as there's no alchemy involved.
Your cost is calculated as such:
The final cost is equal to (modified base cost × Challenge Rating) +
extra costs. The minimum cost for a mechanical trap is (CR × 100) gp.
So you have (1000 - 700) x 1 + 0 = 300g. You could get it down to 100g if you lowered the save DC on the pit to 18 instead of the base 20.
Note that you currently have a CR 0 trap, and the rules require minimum CR of 1. You can get to 1 without increasing the cost by adding average damage (adding 7 average damage increases CR by 1). This would not increase the cost.
What if it really didn't have a cost?
If something really doesn't have a cost, you can make something up. For "digging a hole in the ground", the cost is time. The PCs can do it themselves, or they can hire someone. An untrained labourer costs 1sp per day.
The economy in 3.5 doesn't make a ton of sense, so you can assume the extra cost in a trap vs a hole in the ground is what's required to actually fool anybody into ever falling into it. It's best not to try to expect too much logical consistency on this stuff, becuase unless you use houserules, it doesn't exist.
How Long Does Digging a Hole Take?
Races of the Dragon p.98 has rules for using Profession (Miner) to mine, which you could also use for digging a big hole for your trap. The rules cover how long this would take based on how good a check you can make, with modifiers for different type of ground. You could combine those with the hireling cost to get an idea of what it'd cost to hire a bunch of people in town to dig a 10x10x10' hole.
(Summary version: a medium size miner can empty a 5' cube per day with a DC 15 check. DC 10 would do half of that, and every 5 you beat it over 15 lets you do another 5' cube. Large size works twice as fast, small half as fast. A second medium miner can assist and use aid another to boost the role, while smaller creatures can have more assistance.)
Now that you have your hole, you'd just have to figure out how to conceal it. There's no particular rule for that outside of the trap rules that I know of, but you could make something up.
Either a +44 or a +30, depending on the type of bonus.
Magic Item Basics
- Grants an enhancement bonus on a skill check greater than +30.
- Has a market price above 200,000 gp, not including material costs for armor or weapons, material component- or experience point-based costs, or additional value for intelligent items.
Creating Magic Items with Competence Bonuses
There is a formula for computing the price to create a magic item. Since non-epic magic items have a price below 200,000, use the following formula:
44 x 44 x 100 gp = 193,600 gp.
Conclusion
There are no maximums specifically mentioned for a competence bonus, as there are for enhancement bonuses. The maximum would be limited by gold costs to create. In the realm of enhancement bonuses, there is a maximum stated.
- Competence Maximum: +44
- Enhancement Maximum: +30
Fun Fact
Competence Bonuses Stack with Enhancement Bonuses. So, a +44 Ring of Lies (Bluff), a +30 Ring of False Truths (Bluff), and a casting of the spell Glibness (+30 untyped), would give you, with no other factors involved, a +104 to Bluff. You can trust Bards... right?
Best Answer
The Easy Ways
These aren't exactly what you want because they're not magic items, but these spells can be put sometimes into wands, sometimes into staffs, or into wondrous items.
A number of even higher level options are available, of course, but that list is beyond this abbreviated one's scope as you're looking for the lowest level one available.
The Hard Way
What you're asking for is an extremely high-level--probably epic--magic item. Killing creatures in D&D 3.X is simple; the dead condition is an easily removed status effect that high-level PCs are supposed to casually negate. Capturing things--that is, not killing something but leaving the creature at one's mercy but without options--is a far harder effect to generate. The game resists the very idea because it's not fun; no player wants such power in the DM's hands (because--poof!--there goes the PC forever), and no DM wants such power in the PCs' hands (because--poof!--there goes the plot forever). But it can be done with time and vast wealth.
Then, after crafting and securing the creature, find a place to put the shackled creature. The antimagic field of these shackles will make this difficult. I suggest first using plane shift to locate a secure spot then shackling the creature.
What follows is the suggested end result for a custom magic item incorporating the previous effects.
Shackles of Eternal Confinement (888,000 gp; 5 lbs.)
These adamantine manacles fit any Small to Large creature and, when fastened, give the creature a -8 penalty to its Dexterity while creating an antimagic field to a radius of 5 feet. The DC to slip out of the shackles is 28, but breaking them is nearly impossible, requiring a DC 40 Strength check. Further, a shackled creature can make only untrained Escape Artist skill checks (as if the creature had 0 ranks in the Escape Artist skill) to slip the shackles, and the creature can't make Open Locks skill checks to unfasten them. Finally, a shackled creature is immune to mind-affecting spells and abilities, poison, sleep, paralysis, stunning, disease, death, extra damage from critical hits, nonlethal damage, death from massive damage, ability drain, energy drain, fatigue, exhaustion, damage to physical ability scores, and any effect requiring a Fortitude save unless it is harmless or affects objects. The creature need not breathe, eat, or sleep, and is damaged by cure spells and healed by inflict spells. Strong necromancy; CL 15th; Craft Wondrous Item, antimagic field, lobotomize, ray of clumsiness, veil of undeath; Price 888,000 gp; Weight 5 lbs.
"Wait... What?": Beyond the scope of this question is how any magic item that creates an antimagic field continues functioning when the magic item itself creates an antimagic field. Apparently such items do continue generating their antimagic fields even when the items themselves are in the item-created antimagic field because, well, such items exist in officially published sources sans eratta (e.g. bulwark of antimagic (Dr 118)), and one must assume that activating those items doesn't also immediately deactivate those items. I imagine it's like Magic: The Gathering's White Ward in that the item's own antimagic field doesn't affect the item... um... for some reason (e.g. "A wizard did it"). The above item assumes such a ruling's in place for this item (which is a custom item anyway), making the addition of the shackles of antimagic to the base item possible without having the antimagic field dismissing the item's other effects.
An Assumption: I assume you want the creature conscious so that the capturing creature can interrogate the captured creature or--more likely--gloat over the captured creature's plight. I mean, I would. If that's not a thing--that is, the captured creature need not be conscious--, craft a custom magic item from masterwork manacles (PH 126-7, 128) (50 gp; 2 lbs.) incorporating a continuous effect like the spell feign death [necro] (TB 89) costing 30,050 gp (base 2,000 gp x3 for the spell level x5 for the caster level and +50 gp for the masterwork manacles). The DM must determine how the manacles of the sleeping prisoner (or whatever they'd be called) work--as he must whenever custom items are involved--, but unconscious creatures are always willing, so a downed foe so manacled is pretty much permanently unconscious and doesn't need any of that air, food, or water stuff. Then the capturing creature can simply toss the captured creature into the nearest extradimensional space of choice.