Depends on condition! If it has been abused, neglected, has bad belts, etc., dirty oil, generally much dirt all over it, and doesn't run---not much, except for parts.
If it has been cleaned up, runs, oil rather clean, has good drive belts, and looks good, you will get more for it.
The year can be determined by the engine type: cast iron block, type of drive, and some other things, although some of the older models are still bringing good prices when cleaned up.
there is a Model number stamped into the right top side of the transmission case, which can be used to help in identifying what you have.
Things were changed over the years, and that helps too.
I had a 6 h.p., Tecumseh powered, cast iron block, 2 belts drive, battery electric start, in fair shape. Sold it for $400 5 years ago. Still running!

I recently purchased one 6HP, overall pretty clean, wouldn't start but had good compression and spark so knowing nothing more paid $200.00 for it. But I see them going much higher running and tilling.
Got mine running an hour after getting her home.

A friend of mine got $750 for his, i forget the year model, but it had the original Clinton engine. In fact it was all original, but well taken care of over the years. Saw another old one in Crossville Tn. They were asking $700 for it. Also all original except new tires.

1973 horse for 175.00 2 months ago change all oil 2belt model air filter was dancing in the housing went to parts store while at parts counter guy next to me had one for sale 200.00 follewed him for 4 blocks to look at it final was 180.00 and 2 old tires mounted on different rims engine would not start dry gas in carburator it was single belt 4 speed 1979 model both have 6 hp Tecumseh engine. Still looking for transmission vent on them don't see one so for I think be good ideal not to have pressure are vaccum in case.

When i bought my old horse a couple of years ago i got it for $100, a steal i think. I too couldn't find a vent for the transmission and was told the transmission vented where the handle mounts to the frame. Kinda like a meshed gear where you can loosen the bolt and move the handle to either side. Any way i didn't think that was a very good venting system, so i took the vent from an old Ford rear axle housing, "with the rubber hose on it" drilled and tapped the Horse tranny upper housing, put the vent in and so far i haven't had any more seals leak.