My system's a bit different, although my lack of training would severely limit spending based on the OP's idea. I just avoid buying something where I can't justify the expenditure for the return.
Yes, I have to be able to afford the stuff first. Pay it in full. Yes I use a credit card to buy pretty much everything but it all has to be paid for by the end of the month.
After that, if I can't get something out of it, I won't be able to convince myself to buy it.
Disclaimer: I feel backups are important - bikes, wheels, etc. I'll spread the purchase time out over 4 or 5 years but I'll get a back up if I like a product. If I don't then I'll sell it off.
Also if it works I won't replace it. I have 10 or 15 year old booties; other than the logo peeling off there is no visible wear on them. Therefore I can't justify buying a sleeker pair.
Sponsor equipment opportunities I take. I didn't have a back up helmet (I crashed my primary and was using my backup). So I bought another helmet through the team sponsor. I bought some tires, tubes, tape, some other stuff.
Other rules: You can never have too much bar tape or inner tubes.
You should stock, when opportunity arises, any spare "wear items" (chains, cassettes, tires, brake pads, bar tape, cleats, computer mounts), "consumables" (chain lube, grease, cables, cable ends, cable housing), and "fit items" that you use (saddles, pedals, bars, stems, and frames/fork).
If you get a chance to buy bike shop equipment (typically when one closes down) buy it, especially if you know how to use it. Truing stands, BB/Headset presses, workstands (although I prefer the fork/BB mount portable stands), even benches and such.