returning back to comcast in 2 weeks

will be coming back to comcast after a 18 month long absentence one year with u-verse and using my box and being off air antenna for the last six months would like to know i have seen cable cards on ebay for around 10.00 can i go and purchase my own card and use them in my roamio and premier xl or do i have to order from comcast, am also not sure what my charge will be i have seen from 1.50 to 9.95 per month. can i use my own card and try to save some money or does comcast call the shots curious is all