I'm not sure about others, but I usually tend to consider 'dangerous' overselling at about anything less than $0.50/GB of transfer. I know that my own prices are fairly close to that level, but we've found a business plan that works and doesn't leave us in danger - and no it doesn't even include overselling.
As someone, I believe it was Blue, mentioned previously you can obtain data transfer these days for $0.10 - $0.15/GB from some companies. Obviously, you'd have to sell for a good portion more to maintain profitability and not collapse after the first few clients. There are many various costs in running a business and these have to be accounted for.
I think that another factor is the age and size of the company. Larger companies are able to purchase more and in bulk which often results in discounts when then in turn would transfer to lower costs (depending upon the company). If I know the company is a reseller, or know that they are very small and not some large corporation (IE: 1&1) and they're selling packages for $0.25/GB then I'd consider that overselling just given the current prices. To make a decent profit, you'd have to oversell the server to a good extent to stay in business.