There are big dips, buy not related to time since release. I guess it's more about when a new version comes out. Anyway - you can verify whether that happened or not from the chart.
If you are shopping for a bit of a CPU than newer models, make sure that the motherboard that fits your requirements is also available. Motherboards tend to get more expensive 2nd hand once they stop being manufactured, as opposed to CPUs.
There are big dips, buy not related to time since release. I guess it's more about when a new version comes out. Anyway - you can verify whether that happened or not from the chart.