Dorne would probably happily pay for it once we bring Elia back.
But seriously, you're missing the point. Elia was a Martell and thus falls under Dorne's control.
Rhaenys, on the other hand, is a Targaryen. Yes, Elia is her mother, and Oberyn and Doran are her uncles, but ultimately she falls under our control since we're the head of House Targaryen.
We wouldn't be bringing her back for Dorne (like hell I'd give her to Dorne), we'd be bringing her back for us, because she's our family and she's a baby dragonlord/mage in the making.
It's not going to be any time soon, don't worry about that, and I have no issue with making Dorne pay for it, but I definitely intend to try at some point.