On page 284 in the DM's Workshop there is a section on Modifying an Item. It talks about two types of modification, substitution and fusing it with other items. However it not presented in terms of the PCs doing the modification but the referee doing the modification and then determining the resulting rarity.
The default case is that you look up the magic item you want to make, look at its rarity which will tell you the cost to make and the time to make it. That all the rules addresses specifically.
With that being said, there are some tools and common sense rulings you can use when PCs want to modify an existing item.
I recommend doing the following.
- Look at the final item that the players wants to make. Goto page 284 and use the guidelines to determine the final item's rarity.
- Look at the existing item's rarity and give that as a credit for both cost and time. This will help but not be a big benefit due the fact that every level of rarity beyond uncommon is ten times the cost and time of the previous level of rarity.
You need to decide what happens if the item remains at the same level of rarity. I would recommend at this point is to charge the cost and time for the next lowest level of rarity. With the provision that common magic items can't be modified. Once they are made that it. Given that there are only four common magic items (two potions and two spell scrolls) your players should not find this a limitation.
I think there's two reasons why it would make sense for magic item creation costs to be as high or higher than purchase cost, that fit with the idea of D&D and the stories portrayed:
Most of the permanent stuff for sale is ancient
For most of it, it doesn't matter how much gp a caster once spent for it. The magic item for sale in the Bazaar of the Bizarre isn't made by the owner, or anyone he knows. It's made by an ancient wizard, who died centuries ago, and isn't going to see a penny of that money. It was probably looted by adventurers, who then decided they didn't need it all that much, so they sold it for whatever they could get for it and now it's for sale for whatever the item's owner thinks he can get for it.
The reason he asks you for 2000gp, even though you can make it for 2000gp is because A) most of the people who can afford a magic item, could also make one themselves and B) he's now undercutting any bored Wizard who tries to compete with freshly made magic gear. Considering these magic items are practically indestructible and require next to no maintenance, there's eons of time that they've been made in and most are simply still around to be found.
The market for permanent magic items is terrible because the stuff literally lasts forever and there's a whole class of people whose only job is to venture into the wilds, "liberate" the items and then sell them cheap so they can get some more potions for their next "adventure". Sellers are simply responding to this dynamic.
Most of the consumable stuff is made by specialists
The reason your Wizard takes 4 days to write a scroll and spends a 100gp on it is, for a major part, because your Wizard is a firebreathing, lightning throwing, people charming, monster summoning murder-machine and not a scribe.
Probably if you decided to spend 90% of your time learning how to write a Scroll of Burning Hands faster and cheaper than usual, you could also learn to do it in half the time and for half the cost and earn some money selling those scrolls to the other Wizards who don't perfect the art of calligraphy but instead waste their time going out to kill things and take their stuff.
You simply cannot reasonably compete with the people who dedicate their lives to creating consumable magic items and never learn to survive adventures. (And the reason there aren't any rules for doing so is because this is Dungeons & Dragons and any such character would be an NPC, not a player character)
Why you can't make money selling magic items
Ultimately, what it comes down to is this: making a lot of money off of creating and selling magic is boring and not what D&D is about, so the standard rules don't allow for it. The above is just flavoring for why it's like this.
Any experienced DM who can turn "making and selling +1 swords" into a fun play session will have enough experience to tweak (or disregard) the rules so that it works.
Any DM who doesn´t have that level of experience cannot accidentally screw up his game by showing his players how to make loads of money without actually risking their hides in the adventures that the game is about.
It's a win for everyone.
Best Answer
The rules are ambiguous, but I think they lean to "yes."
As you know, it's not explicitly spelled out in the DMG. But I think they lean your way, and here's why:
Unfortunately, there's a decent counter-argument of nearly the same strength: "anyone who assists is a co-creater, and thereby must meet all the creator's requirements. The level requirement is simply called out to make it clear that another spellcaster with the right spell but not of the right level could not assist." Of course, this line of reasoning would moot "if each of them meets the level prerequisite," which I find intolerable.
But it doesn't matter, because it's explicitly your call. (Of course, all things are ultimately your call, but this one's called out as such in the DMG.)
Back it up to the very beginning of "Crafting a Magic Item":
Obviously we all understand the "golden rule" of GM-ery: Make decisions and adjudications that enhance the fun of the adventure when possible. (Repeated at the start of all of Wizards' published adventures, and a good paraphrasing of "The Dungeon Master" from the introduction to the DMG.)
But I'd suggest a broader understanding of published materials: what we purchase in the core set is not (just) a game. It's a system, with a game as a worked example included. The rules presented provide you with a default game playable out of the box, but also everything you need to construct a game after that fashion. Every rule, in my opinion, is preceded by a silent "and here's one way to do ___:"