Right now you can only specify additional cost with a negative integer if the game decides your crafting recipe really needs to be four times as expensive as you wanted. It'd be nice to be able to just flat-out override the calculation and specify the crafting cost upfront.
May I also add that, because the game caches these values (and, indeed, the entire crafting recipes) at the start of the savegame and doesn't update it if you change your crafting recipes in the XML..... right now if you're trying to adjust the prices you have to either get it all right at once (bring a notebook), or start a new game every time you want to see if your prices are reasonable this time. This is......irksome.
Just merged in changes from @giantcabbage: https://github.com/kronosaur/Transcendence/pull/46
This allows the recipe to define either markUp: (10 = 10% of component cost) or totalCost (total cost to craft, excluding components).
This will be available in 1.8 Alpha 2.
@shrike: I've added a ticket that should address your comment: https://ministry.kronosaur.com/record.hexm?id=72408