If someone you knew had six 741s, power supplies for each and the controllers basically a plug in and go setup how much BTC would be a fair trade for them if the deal was done tomorrow?
I know they can be bought new with PSU for around 1k each but it will be months before you get them.
I'd consider paying about 8k for it, off course it also depends on your electricity price, as a higher price would mean you need to pay less for the same hardware to break even