Plenty of good advice above. Only thing I would add is that you have to consider that your average flea market visitor is more of an impulse buyer. They're out to get some breakfast and have a bit of a look around, but most don't have any real purchasing desire.
So, you have to do your best to build desire and that's done through your marketing of course, both visual and practical. Visual marketing is what get's buyers attention when they are walking around. Your pricing is all wrapped up into the practical marketing side, and the way you set that is to understand your environment.
Are there direct (primary) competitors there, are there secondary competitors (sellers in the same genre, but different stock profiles). How far away is the nearest retail shop competitor to you, I mean out side of the market and what are their trading hours.
What is the demographic of the market goers, high socio-economic traffic or low. Factors such as these all bleed into setting a price, because you have to cover your competition and be relevant to your customer demographic to have any chance of success at all.
Forget trying to come up with some equation in your head and thinking you'll be able to get close, you have to get out there are research your market environment.