WRT buying a team/expansion fee, it's called initial investment. The NFL isn't obligated to help an owner buy a team to help him join the NFL, and that's primarily why new owners are left alone to find financing, from what I understand. The idea is that you'll make your money after while and again as has been mentioned, you can sell it to make a profit to someone else.
As for revenue-sharing and new stadiums, a lot of owners built new and expensive stadiums to attract high-rollers to buy luxury boxes and such, that they'd get to put in their pockets directly without sharing. However if the proposal that the NFLPA wants is that luxury box revenue should count towards the cap, then these owners who spent money on new stadiums deserve a higher portion of the revenue pie, ideally enough to cover their stradium debts. That only seems fair.