I try to build my own sleds and other jigs when I can, rather than buying pre-made ones. Sometimes, building myself isn't actually cheaper, but they tend to work more the way I want them to. I have bought a couple pre-made jigs, only to find they didn't quite fit my equipment, or didn't fit at all, or were hard to use in one way or another...so I ended up returning them.
I have been buying metal T-track and miter track. For some things, I will probably keep doing that. However now that I have a router, I am thinking I may pick up a T-track bit and router my own tracks when I don't necessarily need the durability of metal.
When it comes to sandpaper. When I first started out, I was using those pre-cut round pieces, often with the velcro felt on the back. I tried to save as much as I could, as those things are expensive... I've tried ton of brands and types, and they are just expensive. I tried ways to keep them going... I eventually picked up some 8.5x11 sheets of Norton's sandpaper, and it just worked so well! Something about that sandpaper, it just works better than any of the other stuff I've used, except one other kind that also comes in sheets. The Nortons, being sheets, is not as expensive as the circular stuff, even the 2" size...guess you really pay for the cutting, and the extra coatings that stuff usually has, the velcro felt backing, etc. So I've saved by using the Nortons. But then, I also find ways to use the smallest pieces I can at a time as well, either on pens, or on larger turned items.
I've found that sandpaper works exceptionally well when you DON'T try to reuse it. I even researched the subject, and it really doesn't take much to break down the sharp points of the grits, rendering them far less useful. I've found that coated sandpapers might as well be near a full grit coarser than their rating as well. By not reusing any sandpaper, and just being efficient with the pieces I cut, I think I get better, faster sanding with better, smoother, cleaner results. That's been a money-saver that's also been a time-saver and a quality booster.