Doing 3 or 4 dives a day would be a lot better on nitrox but it would workout very expensive as all the O2 is shipped in and the mark ups at each stage are high. See the attached photos for the prices much more expensive than the Red Sea where nitrox is about £3 a fill or even free. Trimix is available at around $4.5/ cubic ft and rebreather divers can be catered for with but they do like some notice. The facilities for mixing gasses are all very good and the guys that do it are well trained which is good to see so if you do want to dive mixed gasses on the deeper wrecks you should have no worries apart from the credit card bill.
One option to keep the costs down and help with the off gassing is to use a 50% O2 mix for the 5m safety stop this is being done by 2 Australians also diving here who are doing some of the deeper wrecks on air. That way the deco cylinder will last for a few dives but you have the advantage of hi O2 deco