The last figures I saw had the Big XII second to the SEC in per-school revenues from the conference.
In TV revenues, only, the latest figures I've seen are from 2014, which showed the average Big XII school at $26.2 million, second to the BiG at $26.5 million. I don't think that includes third-tier revenues, though -- although I'm not sure.
Third-tier is the games that aren't picked up by the networks. The Big XII teams retain rights to those games. This was an accommodation made to allow Texas to keep the LHN, but it worked out well for other members, as well. For example, it's the reason Cyclone fans could watch all their team's men's basketball games live this year while Hawkeye fans could not.
You have to be really careful with comparisons. The third-tier deal is one reason, but there's also the fact that some conferences are just starting new contracts while others are completing contracts that will presumably be increased when they are renewed. So what the numbers were last year might not be the same as what they will be this year or next year. For example, there is one comparison that shows the SEC dead last, and nobody thinks that's an accurate indication.