The Garrett Sea Hunter Mark II is a good one with pretty good discrimination for just over $600. Anything you find cheaper than that I doubt would be nearly as good.
Excal 1000, Excal II, Sovereign GT, CZ-20, Tiger Shark, Tejon, GTI 1500, Surfmaster Pulse, CZ6a, DFX, AT PRO, Fisher 1235, Surf PI Pro, 1280-X, many more because I enjoy learning them. New Garrett Ca
You will get what you pay for. New PI's run about $600, and use are just that, used. You aren't sure what you are getting. Time is the only thing you never get back, why waste it swinging a detector that isn't up for the job at hand.
You should be able to pick up a DetectorPro for about $400. You did say cheap. Many owners buy them for diving and then are not secure with them after the first use and quickly resale at a reduced price.