The two networks signed a deal to keep the show on their networks for five years. No word on how much the two networks paid for the rights to New Girl, but Deadline is reporting it is under the $700,000 per episode Mike and Molly scored from FX last year (it premieres this fall). While that may look low, New Girl also has a $900,000 per episode deal with Netflix.
MTV is a solid fit for New Girl, which targets A18-34 demographics, in particular, female demos. The two original comedies on MTV, Awkward and Faking It, are very similar to New Girl.
As for TBS, the addition is odd. TBS is built on multi-camera comedies, including The Big Bang Theory and upcoming 2 Broke Girls, as well as originals Ground Floor and Sullivan and Son. The sole single-camera original on the network is Cougar Town, and it is on its way out next season. Perhaps the pick-up of new original Tribeca (single-camera starring Rashida Jones) is part of the reason TBS went after the rights for New Girl. In any event, its still a confusing pick-up.