The individual plan for unlimited everything, no data caps, except for 450 minutes to landlines, with a per-minute charge thereafter is $70/month. For unlimited everything , it's $100/month.
Correct me if I'm wrong, but your Nexus One plan at $70/month is capped at 500 minutes and does have a data cap, does it not?
Further, as an EVO owner, I can assure you that Sprint adds a $10 premium stated for 4G access (whether you use it or not) and with some suggested preferential treatments for any data throttling.
That puts the EVO at $80/month compared to your $70. Sprint charges $30/month for the feature that lets your phone act a mobile wifi hotspot - it is not required if you don't use the service, and if you do want that capability but would rather escape the charges, you simply void your warranty, root your phone and do it for free.
Bottom line, Sprint's pricing isn't mysterious, it's $10/month more to use an EVO on Sprint than a Nexus One on T-Mobile with plans that are only arguably comparable.
I'm not saying that I'm happy with the price, tho.... ;)