Can someone give me very easy to understand examples? And how to calculate?
Suppose someone views a page off of my web server. This page is 400kb. How much is the bandwidth in and out for me?
In this context:
The majority of hosting services are heavy on the outbound bandwidth (delivering content), and lighter on the inbound bandwidth (content requests). Conversely, ISPs are heavy users of inbound bandwidth (content being delivered).
Bandwidth is a datarate, not a quantity of data.
Assuming you mean How much data transfer quota does this cost?, the data out totals 400kb plus http header overhead. Data in is the sum of the URLs needed to retrieve items from your site, unless your host doesn't charge for URLs.
Well, bandwidth in is lets guesstimate ... 10 - 20 kbyte for the HTTP request headers for all the resources (images, css, js etc) that are loaded to form the page. Bandwidth out is 400 kbyte as you wrote.
"In" is to Rackspace's servers, and "out" is the other way, i.e. to the Internet / user.
"How to calculate?" Well, that depends. What metrics do you have for your current traffic?