These machines are twenty years old and pricing on them is literally subject to the daily whims of children and collectors, and the supply is by no means "steady" at this point -- that's up to institutions that held onto them and just now want to get rid of them, individuals who previously collected them wanting to move on or lighten the load, or home users who bought them new doing the same.

There's realistically no way at this point you can say "the average price of an SE is $10" -- and even if you did compile a huge list of comps and assembled a moving average price projection using some statistics software (or Excel and plug-ins, or Excel and some know-how) then the list wouldn't be useful as a guarantee that somebody will be able to find $MACHINE at $SPECIFIC_PRICE_POINT.