GB lifetime = $112 usd
WoW fully upgraded = $119.97 usd (free month used for leveling to 85)
WoW monthly sub = 14.99
$231.97 initial investment, 14.99 recurring
Gold = $2.8 per 1k at IGE.com ... I consider this the most expensive place to purchase gold, you can get cheaper elsewhere and even cheaper with a private seller.
You'll need to farm 82k to break even and an additional 5.4k per month.
Keep in mind that you do have to level that account to 85 and that in itself is a risk. If you happen to get banned, you'll need to invest another $119 to restart, about 43k gold. Also there is an ENORMOUS time investment between setting up the bot and selling your goods.
In the best possible scenario you should make back your initial investment after about 2 months...everything after that is pretty much profit.
In the real world, I'll use myself as an example. I've been botting for several months now...about 4 or 5 months. I used HB to level one of my mains to 85. I have a 2nd account that I consider a main but is also disposable (I plan to just level different toons on this account and xfer to the main account. And finally, I have 3 farming bots...2 at 80 that are now on hold and 1 at 84.
At this point, I have yet to break even on even 1 farming account. This is partially due to the complexity of running multiple bots and all of the Cata patches that screwed HB. Most of the time I've spent trying to build profiles and generally tinkering with the bots in different areas. I've been 72hr suspended on 4 accounts because of grinding so I refuse to run in grind mode for more than 6hrs at a time.
So to anyone considering buy HB/GB to supplement your gold income. Unless you're in it for the long haul, DON'T DO IT! You'll need several months to break even and even more if you happen to get hit with a ban.
If you're interested in selling gold for profit...GO BIG or go home. This means you'll need to invest probably close to a thousand (assuming you already have a good PC) and more if you need to purchase hardware. Don't think you'll be able to sell gold on the side because you won't and will end up spending more than you will ever make.
Go for HB/GB if you like to tinker with bots or don't care about losing an account or two...this is me. I actually started out as a multiboxer and turned to multibotter. I just have fun with it and it's been providing me and a few friends with some spare gold and mats to play with.