The cheapest place I've seen is amazon. Going Rogue is $30 on amazon, but it comes with a month of play time - which effectively makes the game $15, and 1 month of play $15. They also sell 60-day timecards for $25, which is $5 off.
BTW, I love City of Heroes/Villains, have played off and on for 6 years. I don't have tons of time to play it lately, but I'll probably stay subscribed for however much longer it lasts. Definitely my all time favorite MMORPG.