By the way, it sounds like you've got a couple of unused licenses.
You do not need to buy a sql server or windows server license for the passive server - either in log shipping or clustering. Unless of course you're using the SQL Server for other, non-passive, purposes.
Taken from Microsoft's licensing white paper:
"When doing failover support, a server is designated as the passive server. The purpose of the passive server is to absorb the data and information held in another server that fails. A passive server does not need a license, provided that the number of processors in the passive server is equal or less than those of the active server. The passive server can take the duties of the active server for 30 days. Afterward, it must be licensed accordingly."