I haven't checked prices at the farmer's market, but on craigslist around here they sell for $4 or $5 a dozen. Sounds overpriced to me, but maybe because they are all advertised as organic so the feed might cost more. I would think that $3 a dozen is equivalent to 25 cents per egg and sounds reasonable if you just want to sell some excess eggs. But if you want to sell more, then the price should be lower (maybe $2.50).
Another approach is to give discounts for larger orders (e.g. $3 for first dozen and $2.50 for each additional dozen). Or set a price of $3 for a dozen and $4 for 18. Or start a frequent buyer's discount where the 10th one is free (this is really the same as a 10% discount but gets people to come back).
I would start testing at the higher price and go from there. Of course if you are just looking to cover the cost of food and not make a profit or make up for startup costs, etc., then calculate it that way. Actually, that is a good place to start anyway. For instance, if a 50 lb bag of feed costs about $16, and 1 chicken eats 1.5 lbs per week and gives 6 eggs, then you are looking at about $1 per dozen (16/50*1.5*2 = 0.96). Factor in the fact that you might get 4 or 5 eggs instead of 6 and you can adjust it up to maybe $1.50. Make a spreadsheet and factor in anything you want to come up with a reasonable number.