Decent idea.

I'd say a system like the beast tribes would work best. You're given materials to make replica items of no value with equal difficulty to 1, 2 and 3 star crafts. I don't think it should cost scrips or lots of gil as this defeats the purpose of being an ingame alternative to third party websites, which are free.

Plenty of crafters actually have no idea third party websites exist, it would be a good way for them to learn.