I personally know of three WH20's that sold for under $1,500 in the last 3 months. All games had nice side art with little to no fade, a couple cracked boulders, and cracked ramps at the entrance. All had a little wear outside the lost mine, but little to no insert issues.
The longer you wait, you cheaper you can find games. Most people on pinside seem to want them now, which is fine. But just because you bought the game now instead of waiting for a deal, it doesn't mean the game is worth what you paid.
Also, on the other side of coin, just because you can buy WH20 for $1.5K with a little patience, doesn't mean you can't get $2,200 for it if you sold it.
I paid over $2K for my Congo. Since then I have been offered a very nice one for $1.3K. Is my Congo worth what I paid, no. Is my Congo worth more than $1.3K, yes.
Though, the statement I hate the MOST is "I wouldn't sell mine for $xxxx" What you would sell a game for has nothing to do with anything, it's what someone will buy it for that matters.
Sit on a game long enough, you'll get a good price. Wait long enough for a game, you'll get a good price.
Pinball's aren't going up in price, people are just getting less patient.