Sure, they might be overselling if they charge $10/month. The main focus of my post was to demonstrate the extremities at which it happens and that you shouldn't be fooled by low prices and/or high figures.

There's no magic calculation to detect overselling, it really depends on server specifications and the resources available to the host. Disk space is the key commodity here, so works out the most expensive because it's limited and is the first thing to run short. One thing I didn't mention was that the more disk space used, the more backup space is also utilised, so it's a knock-on effect. 1GB of useable data becomes potentially 2GB or 3GB you need to store for disaster recovery - this all adds up. If you backup multiple times a day as some hosts do, it can become even greater.

In a non-oversold environment, I'd expect 1GB disk space and somewhere around 20-40GB bandwidth to cost between $10 and $20, depending upon the host. Just my experience, though.