$7-900 for an SVT-IV, $400-600 for an SVT-III Pro. Be careful, there are also SVT-III (not Pro) heads out there, make sure you know what you're buying!
I just paid $450 for my 3Pro shipped. I also (a couple years ago) got a like new 4Pro for only $550 - but I don't think that that is a typical... Generally in the $700 - $900 range for these.
My first amp ever was actually that very thing... Nobody bothered to tell me... Like I even knew the difference, though!
Any amp is only worth what your willing to pay for it. Your apparently willing to pay for an Ampeg. Damn good choice! I think your safe to pay anything under 650.00 for a used SVT3Pro, providing it has substantial tube life left. You did not mention your budget. If $ is not an issue, buy a new one and get the industries best 5 year warranty. That way you could live and play with no worries. Good luck.
I paid $525 for my 4 yr old SVT 3 Pro. It was kinda scratched up, and needed a fan replacement. I just put 5 new Electro Harmonix tubes in it and using it as a preamp only it sounded nice and clean. I'm anxious to hear how the full head sounds in my cab.