xbox live subscription
I was able to play Halo 3 on XBOX Live over this past winter holiday, with my three sons (over 13, of course) and it was absolute fun! I will be getting an XBOX 360 soon, but likely by a PS3 just to get the Blu-Ray functionality. Most opinions I have read say that online gaming for PS3 is rather lame compared to XBOX Live.
Also, the XBOX live subscription is roughly $60 (US) per year, not $50/month.
(See http://www.xbox.com/en-US/live/memberships/12month-premiumgold.htm)