No guide for old bike prices as the condition can vary too much. Bike like that's got to be worth £3500 all day long and £4000 on a nice sunny day...
No such thing as average mileage on a classic as condition, history, extras, running etc count for more. Then again, it would be a real optimist who'd believe a Smiths/Veglia set up would still be running after forty years of Meriden's finest vibrations.
A bit late for my tastes, but there's always a market for a good T140 as the spares are available, dead easy to work on and a good ride too.