Crafted weapons and armor scale with level, that includes the price. At level 15 I can get over 1000g from each item sold, and the materials only cost like 4g. The longer you sit on your stockpile the more it's worth.
The kits are basically crafted unique items. You find their upgrades in pre-set places throughout the game, though it will still take some crafting skill to put them together. Honestly though they are pretty disappointing, and you'll generally find basic crafted weapons do more damage AND have better attributes, seeing as the upgradable kits can't be enhanced in any additional way. If you get them early they might be handy in the beginning of the game but the upgrades for them suck.