These fork-lift upgrades...
I don't have the specific info on the latest Power 6 to Power 7 uplift, but it has fit a defined framework in the past...
They have been priced so that the tax-writedown for the old-box is higher than this upgrade.
Imagine a typical 4-year writedown (after six months of usage) for a $200K Power 6 box. After one year, you still have 30 months of depreciation on the 'books'. In this case, $25K has been written down, $175K remains to be done. IBM might sell the upgrade for $150K, PLUS any memory and I/O changes. It can be cheaper than a new purchase, but it is rarely cheaper than buying-a-used-machine and upgrading it.