I reckon the BBC are holding all the cards when it comes to remuneration. They have a hit show, yielding great ratings and, (I'm guessing), loads of celebs and probably pros, (though maybe not from right at the top of the dancesport tree), queuing up to be on the show.
Given this, they will be able to dictate the market rate. I'm fine with this. The exposure the celebs/pros receive and the heightened profiles that result must be worth a great deal. So their direct pay might be considered a tad miserly, but all the fringe benefits must at the very least double, triple or quadruple it.