I never buy substrate from fish stores unless it's a specialty substrate like Eco-complete and it's on sale or something. Other than that, I buy concrete/landscaping gravel or concrete/pool filter sand.
I never use filter cartridges. I just get sponges and floss and rinse them. They can be used for a couple years.
I also try to build my own stuff like dividers instead of buying the pre-made versions. For the price of one smaller one, I can make one 55 gallon size, one 75 gallon size, and still have materials left over
and it's true, being "in the trade" helps. I get normal supplies 30% off, normally fish 50%, and then I also know people who get me good deals like a new, unopened Fluval 305 for $50...