Other than dallies, trade skill cooldowns, and some quest rewards like argent tournament stuff, I haven't really found anything gives me an advantage from multiboxing. I think dallies are really the only thing where I can proportionately make money faster than a single toon (probably do them 30-40% quicker than one toon and the rewards are exactly 1:1 for each toon, so overall a minor advantage).

Some of the old raids/instances can give good money for a run, but there are lockouts and to be honest I've never found the gold/hour to be as good as some people claim. Farming is ok, but the gold per hour per toon is rarely any better than playing one character. I think some boxers tend to see a lot of gold and overlook the fact that it has to be split over several toons.

My current strategy is taking advantage of 5x dallies and putting more money into the AH, since I can raise it so much faster. Additionally if you have extra 80s which you aren't gearing up or enchanting, they can be used to fund the characters you do play.