this is the cheapest surfable boat i found http://adcache.boattraderonline.com/6/9/7/90358797.htm i have a 1999 malibu response and surf behind it, just make sure it has at least a 320 monsoon engine so it can go when you weight it down, not the best wake, but it is surfable
Either the Riders or Sunsports are both going to surf great. You should be able to find something in your price range in that boat. Just be sure to look at the stringers.
I had a 88 Supra Mariah and a 87 sunsetter both of which can be had for under 6K--sold the sunsetter with a new engine for $5500--both have solid wakes and respond well to ballast