What is the cost for hosting a website?

I am doing budgeting for a start-up social network. I am looking at a 1 to 2 year horizon. Does anyone have a rough estimate of how much the server cost will be? Let’s say at monthly page hits of: 10000, 50000, 100000, 500000, 2000000. The website content will be limited to just profile photos and possibly resumes.