It depends which CPUs we're talking about. AMD prices tend to drop pretty significantly as new ones are released. Intel prices sued to drop for new tech but now stay pretty stable. Sometimes they drop $20 when the next big thing comes out.
Intel CPUs generally hold their values. They do go down overtime, but not by much. For example, I think the Core i5-2500k had an initial release price of $235. It is currently selling for $220.
Over the past several years, AMD CPUs have decreased more than Intel CPUs because of overall lower performance. In think AMD's FX-9xxx series dropped by over $400 - $500 from the initial prices. The FX-8150 was originally released at $240 or $245 and currently sells for $200. The FX-8350 was released for about $235 I think and sells for $195 - $200 now.