This is a good way of doing it. They'll have to understand that you cant afford it and if they where nice enough to build you a coop then they should be nice enough to accept your changes. I sell my eggs for much more than other Free Range eggs Around the area and much more than the store bought...
1# Do you have any other neighbors or people at work that you could sell them to.
2# You could also just ask them to buy a bag of feed between them every so often.
3# If they are nice neighbors (Like mine) you should just be able to say it straight up.