You use your materials once: mainly simply because it doesn't specify otherwise.

Any spell that specifies a single target and has a range greater than touch can be chained so as to affect that primary target normally, then arc to a number of secondary targets equal to your caster level (maximum 20).

Chain Spell involves casting your spell once on one target, and then the effect arcs from target to target. There's no reason it should require casting your spell multiple times, or sacrificing the resources equivalent to doing so.

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.