I recently had a thread with this same topic -- with many helpful suggestions made. One thing to consider in making these bags is that you want to find a source of zip pulls compatible with the zipper tapes -- if you can find the pulls, you have 2 zips for the price of one. If your source of extra pulls is incompatible with the tapes, then you are wasting half the zipper. So the cheapest zipper may not be the most economical.
Some stores sell zippers by the yard -- you can put multiple pulls on the yardage you buy. They are almost always basic colors like black, white and beige, but you do get a source of lots of pulls this way.