Depends on what type of PPC you're running.
If you're testing Google Search or Facebook, $50-100/day minimum. Below that and you won't get enough data to make real decisions. Most campaigns need 50-100 conversions before you know if it's working.
If you're testing popup or push traffic (PropellerAds, Adsterra, RichAds), you can start smaller — $50-100 total per campaign test. Traffic is cheaper so you get data faster.
My advice: Pick ONE traffic source and ONE offer type to start. Test with $200-300 total budget split across 3-4 variations (different GEOs or angles). Most people fail because they spread $100 across 10 different tests and none of them get enough volume to matter.
Track everything from day 1. If something's bleeding money after 100-150 clicks with zero conversions, kill it. Don't fall in love with a campaign that's not working.
What vertical are you testing?