I've seen this issue come up on a variety of other forums and although I know it was cleared up in the xplay thread, I am making this explicit post to help make sure it isn't missed by new visitors:
Sins costs nothing to play on-line.
Here is the xplay segment transcript:
Morgan: "This is going to destroy lives, isn't it?"
Brad: "Yea, but you won't have to pay $15 a month for the service. (i.e. unlike MMOs)."
On-line play is free, unlimited.
Please spread the word in other forums! And use a SINS banner 
Thanks 