Definitely check out pawn shops. A Traynor 1x12 tube combo showed up around here not long ago, just under $400, they might take $350 if you have it in you rhand...I passed up another Traynor 2x12 80 watt combo when I got my Cort Cl 1500, should have grabbed it and tripled my money, it was that cheap, along with the Cort.
Peavey's Classic 30 and 50 are good, I get not enough bass from the 30 though, and have to open it up to swap out a cap or two to get a good sounding amp.
Some of the other Peavey tube amps do quite well, the 5150 won't be that cheap even used, but it also doesn't have a reputation for great cleans.
Fender will probably be fairly unlikely, I've seen a fairly beat up Twin Reverb go for 8 bills...70's pre-master volume will be over $1000. Ditto for Super Reverb, Vibrolux, Blues Deluxe, you name it. The older ones are not getting cheaper. Newer ones might be found used in that range, they seem to be hit and miss. Some sound good, some just another amp.
I didn't htink to look at what part of the country you're in, but some locations have lots of good equipment in pawn shops. Unfortunately it's pretty limited around here, but some nice goodies turn up from time to time. I checked out a late 60's or early 70's Ampeg a couple of years ago, sales guy said it didn't work and I almost made an offer since I'd already seen the speaker wires dangling...
but I already have 5 amps...yeah but it's a vintage Ampeg...yeah but I have 5 amps...one's a vintage Super Reverb...yeah but it's a vintage Ampeg....geez...decisions decisions...