lol i created a modlet to append the item_modifier.xml with economic prices, didn't work. in testing, i realized they already had sell prices, except the muzzle break, offhand i didnt see any data in the csharp dll relating to this. and the modlet i made did not change the price.
hours wasted, but hey, it was fun.
so sorry, no luck on my end.
i assume your goal is to increase the price of the mods?