her butt went for $30, arm $10
If your target market is mid-20's guys, I think this angle could have some merit. If your target market is the "average" consumer or corporation, I'm not sure how much marketshare you are really going to get from a pair of tits or an ass logo.
I think the real angle for these girls is to do a "Social Media Starter Kit". This T-Shirt girl would be better off not just "mentioning" the product and flickr and twitter, but setting up accounts for these companies and "seeding" the social media sites, then handing the keys over with a bit of training to the organizations. Basically teach the companies that want to leverage social media but don't know where to start how to jump in.
That would likely be a week-long effort, and she could probably charge $3000/wk, based on what I've seen from the current crop of wanna-be SEO hacks.
$75 is pretty cheap, but the in-person exposure is worth $0, so what she is really selling is her ability to promote you online.
It's a novel idea, but really not terribly innovative, although I give her credit for making herself a business in a town that probably doesn't have a ton of employment opportunities for her. The hardest thing for her to overcome will be the fact that people tend to tune out paid shills like this.
She would be better off building up an online persona and name and then more gradually easing into this. OR, make the site "gangofgirlsinyourshirt.com". Sign up franchisees at top colleges across the US, and for $1000/day 50 hot girls promote your brand for the day.