My chickens get bagged up regular old feed from the store BUT to off set the cost, I do the following....
1. feed the leftovers that my kids don't eat (dry cereal, pasta, bread, veggies, whatever) (No extra cost, I'd be buying it anyway)
2. call the local grocery store and ask for over-ripe produce. (They give it to me free, depending on who is working, I can get upwards of 40lbs. Some I have to compost right away but some is absolutely wonderful!) (free minus the cost of gas to go pick it up)
3. I don't do this, but you could... Have the local cops call you when someone reports fresh roadkill (deer in our area). Go pick it up, process it, cook it and feed it to the chickens. Doesn't get more "organic" than that
(Free but you have to put in a lot of effort.)
4. I planted sunflowers this summer with the intention of drying the heads and feeding the seeds to the chickens this winter. (cost- packet of seeds, around $1.50)
5. let them roam in the farm fields nearby once the crops are out, they can eat the corn that's fallen to the ground.