I've dreamed of a medium format digital back since two years ago. I found the unsed unit price is following the trend of dropping 2x each year.
Very recently, ebay has several 36mm x 36mm / 16MB digital backs got skipped with the price ~$3000. A year ago the similar unit would be sold for ~$7500~$9000.
I can understand why 36mm x 36mm is not so much desired, as it is too close to the 36mm x 24mm FF. In many way a 35mm FF DSLR would be much cheaper and convenient, body-wise and system-wise.
Even the 48mmx 36mm MFDB is dropping the price. One without built-in flash memory slot is now below $5000 (and still likely to be skipped) . So it's about 2x more expensive than the 36mmx36mm.
With this trend, I'm expecting affordable (i.e., close to a high end FF DSLR) 48mm x 36mm MFDB in the used market by the end of 2009.
I think medium format camera will quickly pick up the volumn after that.