I get a lot of questions regarding licenses for WSS 3.0 and MOSS 2007. Its not that hard. Software License for WSS 3.0 is free when you use Microsoft Windows Server 2003 and you will need a license per server for each MOSS 2007 server. Next to that you have to pay for each user which uniquely authenticate. And there is even the possibility to buy off licenses when you use MOSS for publishing sites (internet sites).
At the Microsoft website there is a handy tool called License Advisor which can help you for Volume licensing determining the cost and needs. It gives you an idea of how much your/customer environment has to pay. It also tells you which license model is the best.
Have fun!! :)
http://www.microsoft.com/licensing/mla/default.aspx