What is a good Pi detector that is in the 3-400 range? How does the aPI detector differ form a vlf? what is the actual process that occurs when PI or vlf works?
wikipedia metal detectors and you'll get a great, easy to understand explanation of the difference between VLF and PI.
In short, VLF detectors are affected by minerals and conductive salt (ocean water), however they have the ability to discriminate unwanted targets. For example, you can disc out iron and junk so you only dig good stuff.
PI detectors are impervious to the effects of salt water, but they can't discriminate. Most units simply give one tone, regardless of the target. This works ok when gold nugget hunting, water hunting or relic hunting, but bring it to a popular tourist beach or park, and you'll go nuts digging all the trash. For example, at my beaches, I can't go more than 18" without hitting some sort of iron debris, nail, piece of tin foil or shredded aluminum can.
Anyhow, the technology used to make a PI makes it incredibly sensitive to small mass metals. In that price range, I'd look for a used Tesoro Sand Shark, Garrett Sea Hunter 2, Whites Surf PI Pro, Whites Surf PI Dual Field or Whites TDI. Of those, the Dual Field and TDI may be just out of your price range, but both have an adjustable pulse delay which is a special control to further tune the detector to very fine/small gold.
You may also want to go the buddy's section and see if anyone in your area has a machine they'll demonstrate for you. Then if you have samples of what you are hoping to find with the machine, you can bring them, to see if they machine will in fact scan and recognize them. Just an idea.
For a full list of available machines, go the beach section and click on this link.
http://metaldetectingforum.com/showthread.php?t=248905