Taxi would probably cost you something like 200aed ( +/-40€ ), count 1h30 from DXB to AUH maximum. If flying F/J, local airlines arrange drop-off nearly everywhere in the country, otherwise you might find buses ( royal pain, avoid !!! not even sure they will take you directly to AUH ). Been living here for years but no idea if there's any shuttle. Taxi is still the best option, cheapest option would be to rent a car and return it in AUH, approx. cost 70 to 100aed.