I am not sure if this helps as a reference point but we actually did a transfer on the reverse route, Abu Dhabi city to Dxb airport. We just asked our hotel in Abu Dhabi about the rates and after some haggling, managed to get a car for about 400 dirhams arranged by someone at the concierge. These personnel at the concierge don't get much (or anything) if the hotel car is provided so you can definitely haggle with them and be sure that they will have their own private people to do these transfers. (I am sure they get a commission!)
While 400 dirhams is still expensive, it was for an MPV as we had a lot of luggage. A regular sedan car could likely cost a little less.
It was a comfortable 2hr journey and if flight options are better either in terms of pricing or availability from the other city, it is still a viable option to do the transfer between the cities. Just take note of peak hour traffic.
Cheers!