Most multiboxers who play relatively consistently are swimming in money, after you max out mount skills, professions, enchants, gems and glyphs it just starts building up and there is nothing to do with it apart from buying expensive toys.

In my case I make ~2k from 10 alchemists a day consistently, then i farm bt/maggy/gruul/kael and do a daily or two here and there but usually just the gurubashi one for some potential world pvp fun. That's an easy 20k a week at least for maybe 3 hours of gametime. TBC instances drop greens that i turn into mongos, they sell for loads on my AH.

Edit: I have fun doing that stuff aswell, it's not like i'm going out of my way to farm old content.