According to this graph on Reverb, most sold at an average of $3,250.00 or so.
https://reverb.com/p/mesa-boogie-mesa-boogie-mark-ii-c-plus-1984?ended_listing=4096791&utm_source=google&utm_medium=cpc&utm_campaign=340019126&utm_content=campaignid=340019126_adgroupid=25774707806_keyword=_device=c_adposition=_matchtype=b_creative=454209915812&gclid=EAIaIQobChMIg__atZmz7AIVBh6tBh2M4wS3EAAYAiAAEgLKP_D_BwE
They are rare but they'll come on to the market at a rate of about 10-15 a year. I've owned 2 so far but sold them. They are great but I prefer my Mark IV and now my Quad pre amp with the 50/50 power amp. A Mark IIl sounds so close that you could grab one of those until a C+ you like comes along. A III is just more modern sounding but it will get you 90% of the way to a C+, IMO anyway. Whatever you do, get one with the GEQ! It's what makes it tick. The only people who like the non-GEQ amps are people who own non-GEQ amps. They're not metal on their own, you need a pedal with those, something in the loop, but it's not the same as the built-in GEQ. It's close, like the Mark III is close, it gets you 90% of the way there.
I'm sure to own another C+ down the line. The best beast of any amp that all others are compared to. If someone could build a better amp, they would.