The better question is, if pellets are supposed to be a "complete diet" and seeds are not a complete diet, are your birds getting the necessary nutrients that they require from a non-pelleted diet? If you can't answer that, you should probably be feeding pellets.
I've actually noticed that my birds eat less pellets than seeds. My flock of 4 cockatiels eat, on average, less than 30 grams of pellets per day. However, when I feed them seeds, they require at least 45-50 grams of seeds. Seeds have more waste than pellets do, however pellets are more filling than seeds are. Consider how much seed you may be wasting vs how much pellets. Do pellets still seem that expensive?