I've seen estimates of Yankee ownership ranging from 20-25% and the TV deal they signed lasts for many more years. A few years ago it was considered tremendous. Now it is not even remotely as attractive as some of the other deals. The Dodgers deal is dwarfing everyone. The Redsox with 80% ownership of their sports network are much better positioned than the Yanks long term.
The Yankees get $90 mil per year AAV but only own 25% or so of the network.
The Redsox get $60 mil per year but the ownership group owns 80% of the network.
Still, the valuation of the franchises are weighted towards the Yanks due to the bigger market, more seats in the stadium...etc. I think if analysts took a long look at the TV deals though they might want to reconsider. As distribution stratifies via Roku, Chromecast, Apple TV, Hulu Plus, Netflix etc...and cable providers are potentially able to offer ala carte programming more efficiently, networks like NESN will become huge cash cows as regional sports powerhouses.
Consider that "ESPN" is now worth more market cap than "ABC" ( the national network ). It delivers live sports broadcasting that people do not just click away from every time there is a commercial. They charge over $4 per subscriber to your local cable system to carry it, and subscribers demand it. And this is getting replicated all over the world.
To a much lower extent this sort of thing will also happen around the world with NESN. The bulk of that impact is still several years away of course but it's coming. People are leaving cable and flocking to Netflix, Hulu, Chromecast...etc. Internet TV will be HUGE.