Hi Hop,
I feel this is too general a question as posed. Certainly a funky Sony reel to reel in a thrift store is worth less than a Studer 810 or ATR 100. Whether we like it or not ebay has a lot of influence on pricing and it's probably the best place to do pricing research these days.
Most importantly one needs to consider what it will cost to service a machine. A lot of folks selling tape machines are not really savvy as to their mechanical condition. Their "excellent condition", might be your "looks OK and runs, but needs heads, pinchrollers, brakes and one lifter is bent". And so it is usually best to either ask for the option to return the machine if it doesn't perform to your standard, or make an offer that leaves you room to pay for repairs.
I suppose if folks want to we could start a thread on prices paid for various machines. We would have to be very careful to post the condition the machine was in, whether we felt we got our money's worth, and what we may have had to pay for any repairs.