how to price an income stream

I have been offered the purchase of a 40 year lease over a vacant piece of land. The current leaseholder has sublet the land at a much higher rate for 5 years. The sublessee has installed expensive equipment that indicates that they will want to use the land for many years to come.

The leaseholder has priced the sale of the lease on the basis of the 40 years of access but I argue that he only has a 5 year lease and the value of the income stream should be based only on that 5 year sublease.

Any thoughts or comments are welcome

RPiller