I like to think of the machines that need some work as machines that are low in price but high in sweat equity.
Often there's a lot blood sweat and tears in them for me. (the tears are often related to the blood part...)
Here's how I figure worth:
My last 2 machines were $15 and $20 respectively. The $15 was an RAF Singer 15 and the $20 is a 15-91.
The RAF has all of its wiring cut, no pedal, a missing slide plate, a missing stitch length lever and really needs a good cleaning and several adjustments. Normally, I wouldn't buy a machine like this but it IS an RAF with a spoked handwheel to boot!
The 15-91 has failed shellac and is rusty. The motor and light were rewired but I don't usually trust it, so I'll be doing a thorough check and servicing the motor (had a peek inside - needs help)
All told, there will likely be easily 4 hours into the RAF and probably double that on the 15-91 once I finish french polishing it and reassembling. Now, my time is worth something, even if I am working on a hobby. I like to be paid more than minimum wage so factor in my time, parts and "shop materials" and both of those machines are over $100. That's just for the heads. Any sort of case or cabinet should be factored in on top of that.
That's how you can value a machine that's already been taken care of and is ready to sew. If a machine is attic fresh, determine what you would pay for it if it was completely ready to sew, and subtract the parts and the labor you would put into it to bring it up to that standard. That should give you a starting point.
The value is still subjective - what you'd be willing to pay for it - but this helps keep it "reasonable".