If they get 1% of the handle, they can add it to the vig.
Also Cuban is batshit crazy. Chris Christie was just on OTL. He said there is about $190B in illegal sports betting in the US (about $5B in legal sports betting in NV). Even if the leagues get $100B out of that becoming legal and it doubles to $200B, the leagues would net $2B for their 1%.
Let's say football takes half with baseball and the NBA splitting the rest. The NBA would net $500M. Even if the league took nothing (won't ever happen), each team would get about $16M. That's a nice amount of money, but not enough to make a significant bump in the values of teams.
You can bet all the players' associations will want to re-open CBAs if this happens.