I dont think the decline has anything to do with pricing etc.
I think we've simply hit a plateau in terms of upgrade benefit.
For example, when we went from 16mb RAM to 32mb as standard that was a big deal. The same applies for other leaps in the past e.g. smashing 1ghz, going multicore etc.
The current leap is SSDs which don't require an entirely new machine to take advantage of and benefit from.
Add to this the fact that hardware requirements vs software are generally easier to meet theae days than was possible in the past and you have a situation where upgrades arent necessary.