What are the most cost-effective horsepower mods?

Alright, I’ve been digging into my project and keep hitting the same wall budget. I want to squeeze out some extra power without completely draining the bank account. I know the basics like a cold air intake or a cat-back exhaust, but I’m curious what you all have found gives the biggest bang for the buck. Are we talking about a simple ECU tune, or is there something else that’s surprisingly effective without costing a fortune? I’d love to hear what’s worked for you and what might be a waste of cash.