#1 savings for me is reusing other peoples old junk they were going to throw away anyway. From glass to build viv's, window/door trim for frames, chunks of closed cell foam for backgrounds, old scratched aquariums, to metal grating material, if someone is throwing it away and I can think of a use for it...
#2 savings for me is to buy bulk, and usually not from pet stores. I could get 1kg of LECA from a pet store for ~$12, or ~20kg for $30 including tax from the hydroponics store. Same goes for many other supplies, cement for backgrounds, huge bale of peat moss, vinyl tubes for water lines, landscape fabric for covering the LECA is sold in garden sized quantities at Home Depot, etc. It may cost more at the start, but in the long run it is much cheaper. Share the bulk costs with friends if you prefer, or hoard it for future builds like I do.