Anywhere from $10 to $220/month depending on location and a variety of other factors.
The information available online is very limited. The best way to see what's available is probably using their price sheets, which are much more complete than the website. You should be able to pick up a price sheet as well as a channel lineup at a Service Center (check locations and hours at http://www.xfinity.com/support/service-center-locations/), or have Customer Service send these to you.
Use the price sheet as your starting point, and then check the web site, customer service, and this forum for special deals not listed on the price sheet. Shopping their products without the price sheet is like groping in the dark.
Pricing for most packages may have been included with one of your recent bills. Check https://customer.xfinity.com/.