Economic value of multi-box and most cost effective #?

[Ellay]

Multiboxing is powerful doing the following for making gold:
Transmog runs (this would work for old world content)
Gathering nodes (only works on newer expansions, forgot when they enabled a node to be tapped by more than 1 character)
AH sniping - if you like playing the Auction house, you can have a character just sitting at the auction house and snipe the good items, while you can continue to play on your main.
Tradeskilling such as transmutes or similar. It's a lot easier to level up 5 at once, then 1 at a time. So you can get them to max level and do the Transmutes that have 1 day cooldowns.
Emissary rewards - if the reward is 2k, you can knock it out in the same time and make 10k hypothetically.

[Andreauk]

Gathering nodes (only works on newer expansions, forgot when they enabled a node to be tapped by more than 1 character)

All expansions have it apart from Warlords for some weird reason.

[Axgosser]

Probably because of garrisons?

can you make decent money farming old world herbs? XD