Are you sure that's true? He may be working for a cut of album sales instead of based on time in studio.
This is the typical way I understand it...and while U2's situation could be different, I don't see why that would be the case.
Most notable rock producers get a flat rate/salary up front. And often with some prestigious (or whatever term you like) artist like U2, rather than just boost the salary, the producer gets his flat rate and the artists will toss in a royalty of some kind. Royalties have always been complicated to me because of all the varieties, so I won't even attempt to explain - other than to say - there is little chance DM is paid with an hourly wage (to so speak). In other words - more time in the studio (in this sense) is not going to benefit his pocketbook.
And in country/pop/rap/hip-hop it can be different because of how producers skip around. They can be paid by track. So that is more akin to producers being paid for 'time served', where the more time they spend with the artist, the more they get paid because they are literally doing more work.
Also it is not cost prohibitive for U2 - a band that bothered to build their own studio for the comfort and cost saving - to attach payment of the producer to 'time served', when all of their sessions are long as shit anyhow.
But he's probably working with U2 for all of the obvious reasons other than the money. And if you assume that he DOES work for a flat rate/salary, then his working with U2 or literally anyone else makes no monetary difference. DM could make his basic flat rate (whatever he typically demands) producing elsewhere, with The Shins or whomever. However, maybe that royalty is worth it to him - financially - and it might be contingent on finishing up the album. But even in this case, whether they finish in October of 2012 or July of 2013, it's not going to make him MORE money taking longer.
And if all of this is true, and considering how unprofessional it is to negotiate rates after the fact - there is probably a very good chance DM has already been paid his salary (royalties aside).
And since I mentioned the idea of him getting burned out in the studio with U2 (ala Chris Thomas or whatever) I should mention this aside - his home base of operations is in America. So while he's likely not locked up in the studio with U2 for any set length of time or days, he can't exactly go home too easily with the Atlantic ocean in his way. Any trips back home would have to coincide with U2 taking a break themselves. And U2 has taken breaks but I don't really think this album 'kicked off' in full force until Jan/Feb of this year. DM's sticking around (or not) might depend on how far along they were when they started in Jan/Feb of this year. He could stick with them until mid 2013, I just don't know why he would. Because if things are going swimmingly well, then why do they need all of that time? I think it's a case of U2 (or 3/4's of the members) wanting to take the time regardless. To them, what's the rush?
So maybe a financial benefit to him hanging around can be made - about a potential royalty contingent upon completion of the album. After all, U2 are one of the artists that still sell albums - often hard copies (CD). If that is the case, and it very well could be, you could make the argument that it benefits his wallet to hang in there. Without knowing the contract between U2 and Burton, this is all speculation. But there is a reasonable deduction to be made...I refer to my earlier "charming" remarks.