Let us assume that the owner has a reason - presumably a need for money - for the purchase offer, and that this reason does not disappear because of your rejection of the purchase offer, ergo the owner will then sell to a third party. In this case, I would always assume that the new leaseholder - alone because he will presumably want to finance the purchase and repay it as quickly as possible - will in the future fully exploit the adjustment possibilities of the ground lease interest (and then it will evaluate as a flawed calculation to assume the status quo is maintained).
Now let us consider the situation: let us fictitiously assume that the ground lease contract were terminated at the present time. Then the leaseholder would have to compensate you for the value of the building. He would have to offset this against the sales proceeds for himself. Thus, "technically" the valuation is described first, for which it does not matter whether the buyer is you or a third party. The value is therefore "X minus building compensation."
Now you must not make the mistake of equating "X" with the standard land value; rather, X corresponds to the market value of the undeveloped land. The standard land value is not decisive for the buyer insofar as a. it is a valuation parameter not for the buyer’s return expectation but for the financier’s risk assessment and b. regularly, like most other truths, it does not lie arithmetically halfway between the wishes of buyer and seller. New game, new luck: expert index values, no matter how recent, always attest to the past.
From the perspective of the third party as buyer, he would have to clarify his price offer in the following triangle: A. what is the value of the "unencumbered" land, B. the ground lease contract still has remaining term, C. after all, the lease interest still has potential for increase. The uncertainty for you would be whether he raises it and to what extent he fully exploits the scope. The uncertainty for him would be whether you would be willing to terminate early (voluntarily, thus possibly under conditions less favorable from his perspective than those planned for the survival case of expiry).
Whoever has read this far should hopefully understand that there will not be a dozen case studies reported here in two weeks from which usable approximate values could be derived. True to the motto of my old math teacher "mathematical ignorance manifests itself in intensified number crunching," I could only abstractly explain from which points the field of uncertainty is constructed here.
I also hope to have made it clear that "you do not feel the need to act here after all" must not be equated with "so we can comfortably starve the leaseholder with his purchase offer while holding out." You do NOT hold the longer lever - at least not one that is not uncertainly rotten. In my opinion, that would be the ice the donkey prefers if you tried to carry out the wishful thinking that the leaseholder would now retroactively convert the ground lease into a lease purchase and after offsetting you could basically take it over at scrap value.