Mo: its interesting that you have worked out the price of your eggs. I dont think that would be easy with our gang, what with the cockrels and the duck. I'm sure some of our hens only lay when they feel like it, some are really quite old and only lay in the summer. We also supplement their feed with anything from the house, stale bread, veg peelings, fruit that looks a bit dodgy etc.
Our son's doing A level maths, I'll get him on to it!
Ours are organic (Allen and Page feeds) and free range, but so so worth it.