But it certainly isn't contradictory to provide a general incentive, while allowing Dublin scope to increase charges, if necessary.
Even if you were to accept this point, you would need to be sure that the authority responsible for calculating the expenses and spending the money is competent so to do. The DAA does not inspire this sort of confidence. Without going into older history even their last great project T2 was flawed. Every aviation professional I have spoken to (that includes ATC, airline captains and ground staff) say that it was built in the wrong place - all crammed into a congested corner beside the main runway - and that this point was made before it was built. (not to mention that the best they could do for passengers was seats made of cold, hard steel)