Older AMD CPUs may go down in price until supplies dwindle and prices goes back up again. But older Intel CPUs do not really go down in price. You may see them on sale, but that's different compared a permanent or long term price drop. Again, once supplies starts dwindling, the price goes up.
For example, the Q9450 had a street price of about $320 when initially released. I bought mine for around $360 because there was such a high demand for them that e-tailers jacked up the price. Eventually prices settled back down to around $320. Nowadays a brand new Q9450 can be bought for $366...