My idea is this.. when players purchase hours I add in an extra $1 charge (they will be aware of it) (25 hours = $4.15 so I would make it $5.15) and they get $1.00 in tournament credits. They also have the option to purchase tournament credits at any time.
I'm not so sure that's a good idea (imo obviously). People generally don't like being told how to spend their money and the above seems to force them into buying tournament credits (correct me if I'm wrong)
Edit: Im also intrigued about how this game will work out in a MMO format!