If you're primarily looking for prospecting capable detector, you are not going to find anything that will work well in that price range.
Used is gunna be your best bet.
Look for a Fisher GB/2, Tesoro Lobo SuperTRAQ, Whites GMT or MXT Pro, AT Gold, Teknetics G2. Still, you're going to have a really hard time finding find THAT good of deal on one of those in that price range!
Good luck.