I read a study somewhere (sorry, can't remember where but if I find it I will post the link) about feeding flax to your layers boost the omega-3 content of your eggs. I would think that it would be a healthy addition to their diets, even if it didn't do anything for the nutrition of the egg on our end.
There is a lot of hype about flax lately, so no surprise it's expensive at the grocery store. Do you have a health food store locally that sells bulk products? I buy my flax out of the bulk bins at the health food store I work at. I think it's $2-and-some-change per pound right now.
There's a ton of debate about whether to feed flax whole or ground (to humans or animals). I don't know about chickens; I would imagine, out of all of the critters we might feed it to, that the chicken's way of digesting would probably be the best to break down flax seed and I would probably feed them whole. But for yourself, horses, etc., I say GRIND IT. If you've ever gotten a flax seed wet, you will know why. When wet (think, when it contacts saliva/stomach "juices") it forms a very gelatinous, slippery coating (which is why it's so great as an egg substitute in baking). Stomach acid doesn't break this coating or the seed down very well--the whole seeds will literally slip right through you and come out the other end whole. Not to be too graphic, but, um, you can see the evidence of this later if you ever eat them whole (or have fed them to your animals).
The oils in flax seed go rancid very quickly. It's best to store the seed whole and in the fridge or freezer. If you're going to grind it, only grind what you are going to use in a few days and store the ground seed in the fridge. IMO, it's a waste to buy the pre-ground stuff unless it's been kept refrigerated from the start. It's also more expensive. A simple coffee grinder will grind it just fine. Of course, like I said, this is if you're grinding it at all...I don't know about grinding it for chickens.
ETA: here are a couple of links about feeding flax to chickens to boost the omega-3 content of the eggs....
http://news.minnesota.publicradio.org/features/2003/04/30_hetlandc_omegaeggs/
http://www.flaxcouncil.ca/english/index.php?p=food6&mp=food