The franchise itself is iconic...but that's really all that it has going for it now. In Quarter 2 of 2009, 4Kids reported a $0.7 million licensing revenue from TMNT, obviously a very small amount. Now, Mirage probably got a good chunk of that, but really, there's not much money flowing through the franchise right now. The toys are selling pretty poorly and there isn't much other major merchandise except for maybe TMNT: Smash-Up. If this was 1990, then the purchase price would definitely be a ripoff, but I'd say the price is fair given the current state of the franchise. Viacom/Nickelodeon essentially have to rebuild the property by themselves.
As for the money split, Mirage got $50 million, 4Kids got $10 million.