I would offer as much as I could afford, but your bargaining chip is the slightly questionable originality of the amp. For some reason real C+'s go much higher than upgrades.
I have a 60/100w C+ with the serial number 12399 and it's date code is 2/84. BB has looked at pics of the amp and verifies it was a C+ from the factory but that it was probably ordered a C and left the factory as a C+ when it got to production. They are awesome amps, so try to get it if you can.