Amazon Prime users can opt into some of the prime cable channels (HBO, STARX and SHOWTIME) on a subscription basis. Amazon is providing a 7 day free trial to check it out for yourself. After the trial period you be looking at paying $8.99/month for STARZ, $14.99/month for HBO and $8.99/month for SHOWTIME.
If you want to see all the channels available on Amazon Prime click here.