OK...
when you use Small business server, you buy a license for the server.
This gives you windows server OSx Exchange, SQL Server and an IIS license I think.
it's bad practise (dictated by Microsoft) to have these all on one machine, but for some reason they invent this product where they all Have to be on one machine!
after you've bought SBS for the server you have to buy Client Access Licenses (CALS) for all the devices that attach to the sever.
for example you have a server and 10 PC's in your office.
you need 1 SBS license 10 CALs
if you have 20 PCs, you need 1 SBS license, 20 CALS.
on the subject of SBS, it's a pain in the arse,
you can only have 1 DC at a time, which makes it incredibly difficult to upgrade the machine at any time, where usually you'd just have a second DC and migrate roles to it.
you can't do that with SBS cause you can only have 1 DC on the network.
so when/if your server gets old you eiter have to live with it, or revbuild your domain on a new server, and migrate all mail by way of backups, you have to add all machines to the new domain server, (cause SIDS will be different).
Having used SBS (both 2000 and 2003 version) I wouldn't ever advise anyone to go this route.