Something else to consider is your chance of loss. Do you plan (hope?) to do this every year? If so, you should build into your plan the chance that a predator or illness will get in one year and wipe a large number of your flock out. Same thing with figuring food spoilage - you don't want to have to eat the cost if a bag of feed splits open and gets moldy from moisture before you see it. That sort of thing. So figure out how often that might happen (every other year vs once every five years, or whatever), and include the costs you will have already spent but now can't use into your plan so that even when that does happen, your profits for the previous years were good enough that you are not out that money.
I like the idea one poster had about checking your local grocery stores - the good ones, not WalMart - for their pastured chicken costs. Where I live, a pastured chicken costs about $5 per pound - or 20-25 dollars for a full bone-in roasting bird. (An organic one costs much more, but I don't think you said you were doing yours with organic feed.) So if you could charge a bit less than that, you would still be delivering an excellent product at costs below your main competitors (which will keep your customers with you the longest). The good grocery stores in your area have already done the research into what the market around you will bear, so you can just piggyback on their research instead of doing your own.
Does that sort of math make sense to you? If you can undercut the competitor, and still make enough profit that you can accept a typical amount of loss in future years AND still make a profit that is worth the time you spend, then you've hit a nice sweet spot for pricing.