Look at the subway, this runs about $200 and can be damaged by people lifting the playfield without ejecting the balls. The balls crash back into the back of the subway and crack it. Entrance to the Alpha ramp is often cracked, about $100. As others have mentioned, the trough and cannons are known to be flaky. Put the game into multiball modes and make sure it finds the balls.
I think one sold at Allentown for $3400 that looked in good condition, but I don't know the specifics.