How to save money on chicken feed?
A lot of folks see savings by using fermented feed -- you can find some information on that here: http://www.backyardchickens.com/t/645057/fermented-feeds-anyone-using-them
Another thing to consider is how much feed is being lost to "waste" vs. actually being consumed by the birds.....next time you fill the feeder, weigh the amount of feed it holds, knowing the actual amount of feed being gone through in 3-4 days with the number of birds being fed will figure into whether you are experiencing an abnormal amount of feed go out or if you are just seeing the reality of feeding that many birds.
I would suggest considering a treadle feeding system -- this eliminates access to feed by animals you don't intend to feed.
Alternatively, as most small rodent feed thieves prefer to dine at night and since the birds have no need for feed while at roost, you can remove the feed (or, perhaps simpler, cover it) from dusk to dawn