Theoretical Cap on Lease Extension?

I am being offered a flat with just 10 years left on the lease which has a low ground rent. The vendor has served notice on the Freeholder to get the statutory lease extension, which he will transfer to the buyer, and is waiting for a price.
Theoretically, if I buy the flat for £50k, say, must the mathematics of marriage value mean the lease extension can cost no more than £50k as well?