Money a city gives towards a stadium should be returned in the form of shares in the team. Otherwise no deal. St. Louis should be selling their shares to LA for a profit right now...
I think a better alternative is just tax breaks. The team can build a stadium but gets huge tax cuts on everything for a certain period of time to justify the build cost.
I also don't understand why stadiums have such limited lifespans. Wrigely field in nearly 100 years old.