If 7 is actually the last to see the 32bit Windows I imagine the prices for "8" or whatever the following version will be named will then see lower prices since you wouldn't be seeing 2 for 1 editions packaged together. That should at the time start to reflect the $200-$225 price range Home Premium - $250-$270 Ultimate with just a wild guess as you look back at 9X thru XP retail pricing.
With the Vista Ultimate having seen a drop from $399.99 down to $320 now being the older version it will likely see 7 placed at the $399.99 mark.