I do not have credible research to link too, but I'll list what I've heard from someone that does this for a living.
Each MMO is different, this guy I know has been doing this for a living for the last 4 years, he power levels Final Fantasy XI, each char he makes sells for between $200 - $700 USD depending on what items/gold/build it is. He doesn't sell the char directly to players instead he works for a web site that does, he sells the chars to the site and they resell them at a higher cost. He said on avg he makes $6,000 = $10,000 USD per month, and has been for the last 4 years. (This is 8+ hours per day spent leveling chars)
He's said that FF chars sell for about the most of any MMO (except perhaps EVE). Other MMOs he does include WOW, Star Trek online, EVE etc...
The stats he gave me for last year from the site he works with, is that they sold 86,000 accounts last year alone. I believe he meant FF accounts, but could of been different types of games.
WOW max lvl: sells for about $50 - $150
FF max lvl: sells for about $200 - $700
So lets say we believe this: 86,000 * $200 = $17,200,000
And this is just one site that does this, imagine the gold farmers, and items selling etc..
So yes I believe it's very possible that across the world there is $3 billion made per year.
Anyways take this for what it is, just a personal account. I was even considering doing this for a living as it is a way to make money even if you don't fully agree with what it is, it's not fully illegal in all games.