I think smeuser's costs re: MS are about right. If we take Small Business Server 2003, a 5 user licence (standard, OEM) is around £240 (GBP) and Premium is around £540 - less than for SBS2000, but still a per-head cost. This price covers the client access license (with SBS) & so there's no second fee. So £50 - £100 per seat (depending on SBS version) - it may not sound much, but still a lot more than SME (especially as number of users increases).
We supply both SBS and SME servers to clients. We push the SME option to try & beat the competition mostly on price (no per-user license fee, installation is much quicker and Linux can match SBS performance on lesser hardware). We may well be aware of the technical advantages of SME over SBS ourselved, but most of our potential clients only want something to 'do the job' at the best price.
So why do we still supply SBS? Well, some clients have software that requires it. e.g. some server based Accounts package than needs Windows (or MSSQL) to run. Sometimes a two server approach is an option (especially if upgrading from a slow SBS server - the addition of an SME to 'bear the load' & take all but the Windows App off the struggling server so its once again fast enough).
The other factor is Outlook. All our clients use Windows PCs & all use office (& so Outlook). If they want folder sharing, there are ways to do this with SME (using our contrib & a commercial connector) but its not standard SME. So occassionally (in the clients opinion) Exchange is the 'killer app', which again means SBS.