JasonSmith

My 29 with the stock 454 (385hp) mags was really good on fuel. I could go an entire weekend on a tank of fuel.
I would say the twins don't use 1/4 more fuel than a single in the same boat. The added cost of maintenance is negleagble at best. What do you replace yearly anyway besides oil? If you can't afford the additional $25 for 8 quarts of oil, you don't need a boat period.
I will admit that a twin 29 Outlaw is cramped for space in te engine compartment, but there are things yo can do to help yourself out. Buy run-dry impellers. They will last 4+ seasons compared to one season with a regular Merc impeller. Take the bolts out of the back seat and leave them out. Replace all your hard to get to hoses with custom bent stainless tubing to mimic your rubber hoses. Hose changes aren't fun on a 29.
In all seriousness, a twin 29 will cost a few hundred a year more to operate than a single 29 will. Plus, you're gonna be buying a boat (the single 29) that will be hard to get rid of for just the same reasons you are asking about right now.
There are tons of good boats out there right now in the mid 30k range with twins.

JasonSmith

Check out this 33
http://pittsburgh.craigslist.org/boa/2464120394.html
The money is right on this thing, I think.