You need a special version of windows to run MSSQL? In any case, isn't it normally the customer's responsibility to provide licenses for software they install and run?
And yes, it is reasonable to expect the customer to provide the licenses for the software, but hosting companies can also sell monthly software usage to their customers - we ended up choosing a host that did exactly this. The monthly cost we pay for Windows + MSSQL will not reach the total cost of a full MSSQL license for three years.