Interestingly, IMVU had the same issue: many of their target customers didn't have a credit card, and apparently they were able to get mobile phone payment to work for them [www.stanford.edu/class/e140/e140a/handouts/IMVU_Case_Draft.pdf]
But you're right, you need some way of differentiating among your players; some have money and some don't. If you try to charge everyone you lose those who don't want to pay, but if you charge no one then you lose the revenue (and the benefits that would come to the site and for your players if you had revenue) that you'd get from the players who could pay.
What about a premium service? Anything nifty you could offer to paying customers, while the basic game remains free?
But you're right, you need some way of differentiating among your players; some have money and some don't. If you try to charge everyone you lose those who don't want to pay, but if you charge no one then you lose the revenue (and the benefits that would come to the site and for your players if you had revenue) that you'd get from the players who could pay.
What about a premium service? Anything nifty you could offer to paying customers, while the basic game remains free?