I totally agree with Firepower. Prices will rise, not fall. Basic economics with the age old rule of supply versus demand will always apply.

Look at Real Estate. It always rises over time. Prices Always higher.
Look at Classic Cars like 1974 to 1978 Ford 2 door coupes. Prices Always Higher.
Look at anything that is a limited number and of quality build to start with. Prices always higher.

I will say, without trying to offend anyone, most pinballs prior to the late 80's/90's stuff, will not rise as much as the DMD stuff, except a few select titles, because  they are just not as exciting to play for the newer generations of younger kids, but pinball prices will always be higher for late 80's and 90's stuff. To say it will plummet, is certainly not in my realm of thinking, but i could be wrong, it happens a few times every ten years, LOL.

I always say "they are not making any more of them, so get in whilst u can".(my comment excludes the new stuff stern is making, meaning they arent making twilights or indys etc)
Buy up, Buy up.  Smart advice i reckon.      
P.S- if u see the poor state of the stuff getting around germany, u would pay alot less for it too. We are on a great wicket here in OZ.
Hard work in the lucky country takes care of any money issues. Plenty of work n opportunities avail here. Ozzy Ozzzy Ozzy. Ooi Ooi Ooi.