you can still find very good prices on stand-alone '60's Fender Bassman Heads, not the whole head/cab piggyback hardware rig, those are beginning to get expensive, but for a snotty loud, super headroom, handwired clean blackface voiced head, the intrepid 50 watt Bassman head is still one of the better values and just a great hardworking amp.
Peavey Delta Blues with the 15 inch speaker is my fav of the lower priced amps I have played. I got it mint for $300 used. Great clean sounds and when you crank the clean channel nice dirt as well. I ignored the 2nd channel.
Peavey VTM is an excellent bargain that will do just about all eras a Marshall very well
Fender Prosonic is a highly versatile bargain that can be had for around $500-$600 if you are patient
Fender Pro Jr is a great little cheap combo
Carvins are great, quality amps that can be had for cheap
I had an old 100w VK212 from Peavey that sat well between Fender and Marshall -- a very versatile amp, and you could probably score one for $250 or so nowadays.
BK Butler Realtube 6025, got it in a pawnshop for $75, Butler Tube Driver as the preamp stage, SS power stage, one 10" speaker. Yes, it's a hybrid and you might discount it for that ... but it sure didn't feel much like one playing. Very dynamic and tactile.
Scored a used 71 SF Bassman 100w head back in 1991, $100, beautiful cleans, vomity dirt, but with pedal help was my #1 amp for over a decade.
Most Laney amps go for peanuts 2nd hand and many of them are great rock amps.
Old Silverface Fenders, Traynors, Ampegs, certain Peaveys and occasionally Sunn. Sunn amps seems to have jumped in price a bit but I just scored a 68 Solarus for just under 600 Canadian...so like 400-450 USD and that's a hell of a solidly built amp for that kind of cash. Once in a while you can still get a good deal if it's a motivated seller.