It really depends on what satisfy's you condition wise. Some people have silly expectations and want machines that look like they just came out of the box. For 15 and 20 yr old machines, its just not going to happen most of the time.

I saw the writing on the wall 3 yrs ago when machines were very cheap and I snapped up some absolute bargains, saving probably 50 percent on alot of titles. If the machine was a 8.5 ot of 10 or better I would buy it. Sometimnes, if you hang off waiting and waiting and waiting, prices can climb up very high.
It really depends on your absolute deisire and how u rate the game.
A holy grail pin, or just one that you have always been fond of.  I like Nino's idea, buy now, and if you want a minty'er one later, upgrade, but always remember that these things are meant to be played. Are u buying a machine to play, and enjoy, or the cookie cutter pins that u never play, but just show off and say "this one owes me 10 grand". I know alot of collectors who have 5 or 6 $8,000 and 10 grand machines, and have never played them once in 3 or 4 years, and only turn them on to show them off, but no-one ever plays them. To me, its a waste. but each to their own.

I say buy a machine that is 8.5 outta ten and enjoy it yaself and share it. People will enjoy playing a pin that is 8.5 outta ten that is sitting in your house more than a 10 outta 10 machine that wont appear for a good year or two, if at all.  These things are meant to be played, not just looked at. Although both is appealing. Its more fun seeing other people share your titles. They dont care if its a 10 or a 8.5, they just love the new experience of playing a title they havent seen before.