An 80 FLT is probably in the $10,000 range in immaculate (show room) condition and drops from there. All original (original pipes, handle bars, seat, paint, ect) around $8500. Working ,running, daily rider, with matching numbers around $6500. Enough parts to call a motorcycle about $2500.
These are just a few numbers off the top of my head, based on what todays market is doing.
In actuality, it's worth what you are willing to pay.
Personally I'd be more scared of an extreme low miles old bike than a daily rider with 50k+ on the clock. Low miles means it sat for a very long time, allowing seals and gaskets to dry out, the motor to wet sump and tires to rot. Sitting that long will also allow the grease to cake, gas to dry up and crystallize, cylinders to rust and brakes to seize. The more work it needs to get back running the less it'll be worth on the market. I'd have a hard time believing a 31 year old bike with only 5000miles will not need a mess of work unless it sat inside someones livingroom, drained and cleaned regularly.