In a fair market, a producer will look to make 25-50% of the item's value back as profit. This is including labor (which is harder to quantify given everyone has their own idea about what labor is "worth").

For example, if a prism costs 1000 gil to make (assuming the labor value for the mats is already included), a reasonable price on the market would be 1500 gil. Now I say reasonable due to the variances that occur with competition and scarcity (the supply/demand curve in economics still holds true for base values). But for the most part, anything from 1250 - 2000 would fit into this reasonable range to provide profit. Anything over 2000 gil and you start reaching what we call "price gouging." Now it's a free market where price gouging isn't enforced, however, if someone decides that your 13000 gil prism isn't worth 13000 and puts twenty of them up for sale at 2000 gil, you really don't have the right to complain since he/she still falls within the reasonable "profit" margin. Now if this person was selling them for 500 gil a pop, then that's a different story.