It depends on how long they have been seeing each other. There is nothing wrong with a woman paying for date. However if she does it (early only) it usually means one of two things.
1. She is letting him know she does not view their relationship as being a "romantic" one. Therefore she does not want him to "get any ideas" or try to hit on her. Odds are there won't be a second date! Some women also do this by insisting on going "dutch treat"
2. It's also possible she is really "into" him and believes she can (buy) his love or stand out from other women by showing him that she will "take care" of him. In this case she will also let it be known she wants a romantic relationship by flirting or making the first move.