For simplicity, I’m only going to compare Premium because its the lowest cost version that still maintains the main differences between XP and Vista (Aero Glass and Media Center).
Upgrade (Must install on top of XP):
Wal-Mart and Staples: $160
Newegg: $120
Complete (Fresh install):
Sam’s Club: $218
In general, the complete version is going to cost about $100 more than the upgrade. This is most likely because if you went out and bought XP today, you’d pay around $100 for it so Microsoft is getting their money one way or another.