We "run" about ten feeders (It varies with the seasons). You'd think that the cost of seed would monetarily drain us. No! For one thing we use safflower in a few feeders that squirrels can reach. That's because squirrels hate the taste of safflower and leave those feeders alone. We also have a couple of "Squirrel-Buster" feeders that are designed to totally exclude squirrels (no squirrel has gotten a single seed out of a "Squirrel-Buster" in the 18 years we've lived in our house).
We fill feeders when we want to, not when they're almost empty. If a full feeder is your goal, fill it with old nuts and bolts (that's sarcasm). Besides, wild birds get far less than a quarter of their food from feeders anyway.
We strongly advise against buying birdseed from a grocery store or big-box store, whose main interest is low unit cost. Yes - the price per bag is lower, because it's largely made up of cheap seeds that birds in north Texas won't eat (like milo, corn etc.). It seems like it would last longer for the same reason, but that's not so. It actually winds up costing you more because what actually happens is that a bird will pick up a junk seed like milo, realize he doesn't want it, and drop it to the ground. Creatures like rats and mice will pick up these junk seeds; it soon attracts their friends. Result: your feeder gets emptied about as fast as before, you need to buy more seed sooner, and you have a rodent problem.