Who knows what they will do in the future. But in the 5-6 years I've been shooting RFs... the prices have continued to go up. By a lot. New prices have gone up (not just for rangefinder lenses - newer Canon prices are getting up there), which pushes used prices up. I think I got into RFs right before the M8. Certainly the M8 and M9 have created more demand. Lastly, all these mirrorless cameras (Sony NEX, micro 4/3rds, etc.) are also some creating demand, though in my experiences, most of the people using RF lenses on them already had the lenses.

The only saving grace in all of this in my mind is the Zeiss ZM lenses. Yes, they are moderately expensive at around $1000. However, they are fantastic lenses and their prices have been reasonably stable. The Cosina lenses are a great buy, but some of them are getting reasonably expensive now, and some of the grand bargains, like the 28/3.5, are gone.

So who knows if they'll continue to rise. I'd say if you have the money and want to get into them, do it now and enjoy it now.