it’s tricky to say what the Behringer 808/909 will do to prices of the originals. with the Korg Odyssey and MS20, the prices for the originals leveled off. but those were basically manufactured by the original manufacturers, given that David Friend helped with the Odyssey and they got the rights to sell it under the Arp name. the Model D may have impacted Minimoog sales, but it’s hard to say because the Moog reissue came out around the same time and I think that impacted original Minimoog sales more. so… sort of in untreaded waters with these units and the MS101, Bro One, etc… personally, I don’t think Behringer clones will be looked at as “the same” the way the Korg or Moog reissues were. so I don’t expect prices to drop or level off on the originals. just keep climbing like they have. but other clones (Nava, Yocto, Miami, etc…) will get cheaper on the secondhand market, I’d imagine.
either way, I still wouldn’t buy any synth/drum machine as an investment for your future.