I used XP Pro as there was no retail release of MCE, which would obviously be the closest to Home Premium, but your point is taken. XP Pro Retail is £246.75 inc VAT at Scan, Vista Business is £234.52, so my point stands.
And you're right, you can only upgrade SP Pro to Business or Ultimate, but I suspect this is largely down to the feature-sets of Business and Ultimate being matched to those of Pro - would you be happy if you had an XP Pro machine that you used to RDP onto, or had on a domain and tried to upgrade to a version that didn't support those any longer?