Hmm,
John Bantin reckoned in Diver that up to six dives a day would be possible on nitrox:
http://www.divernet.com/travel/04014oz.htm
The fills are 25 Aus dollars per day - less than £10 a day so £60 for the week, not £200.
You're right about the air consumption though - I'll be lucky to get 50 mins from a tank if the max depth is 30m.
The surface interval will be determined only by my buddy (and computer) as diving is unlimited. Might have to see if the DM will come out with me as they will undoubtably be on Nitrox.
The other option of course is solo diving ;-) They allow it after 100 dives, so I'm not quite there yet. Not sure if I really fancy that anyway.