I think a good rule of thumb is to work it out from basics.

• Cost of server (amortized over 4 years)
• Cost of server in server farm
• Cost of communications, phone, IPs etc...
• Cost of qualified staff
• Operating expenses, advertising, insurance etc...
• Rent and taxes

Divide that by the number of users/domains you are prepared to put on a single server.

Add a reasonable profit margin.

With the figures I have for this:
For every US$ 10 a fair return is 3GB of disk space, 45GB of bandwidth +- 15%.
Anything below US$ 8 for the same return something has to give.
Anything below US$ 6 is downright dangerous.

Again - I am in the business of keeping my clients.

NB. That does not mean that a 50MB / 750 MB bandwidth site should cost US$ 0.16. Admin costs and service of a small site are similar to big sites.