This industry is little different to the car industry.
A few years ago I bought a two year old RX8 from a young guy who paid $68K for it new - I paid him $27K - a massive loss to him of $41K in just two years....
A new pin at $10K (depending on the title, number sold originally and popularity of course) would probably be worth around $5~7K after a couple of years IMO - that's just how things work.
Pinballs, like most cars, are not an investment. If you want an investment buy gold or bonds or something and try your luck there.
There is a point (again like cars) where the bottom price is reached - we see this with most SS machines selling for around the $1~3K price point and unless they are trashed that's probably about where they will sit price wise for a very long time to come IMO.
Pinballs are made to be enjoyed so stop worying about a bit of a loss and just play them!