For now stocks are limited partly while this service is trialed. If it runs successfully then stock will increased but will still remain. That's to do with the nature of the money and to avoid annoyingly long waits for the customers. The service is meant to be quick and easy.

As for increased avatars there isn't exactly a rate. Each quote is customized to the details of the avatar.
So if you can tell me the width, height and if it is a gif or not I can get you an estimate.
(Toriprime at least is required for this service but it is substantially cheaper if you have Vip too)