I attended a meeting of the fairly short-lived Westminster Branch of the Supporters' Club back in January 2003 at which David Bernstein spoke for nearly three hours. This was about six weeks before he quit as chairman and six months before we took up occupation of the new stadium, and the prospective move was the biggest topic of the evening.
Bernstein confirmed that the lease contains an option for MCFC to acquire the freehold at an option price that reduces over time because the rent we've already paid can be offset against it. He also suggested that it wouldn't make financial sense for City to exercise the option for at least twenty years, maybe more. In theory, of course, the financial landscape changed completely with the 2008 takeover, and what the club could reasonably afford was transformed but, for reasons that are well known, we haven't elected to exercise the option.
The provisions of the lease aren't in the public domain, but the lease is treated for accounting purposes as a finance lease (i.e. a type of lease which presupposes a change of ownership when the lease terminates). We can probably, therefore, reasonably infer that the buyout figure isn't going to be a million miles from the future liabilities recorded in the accounts as attributable to the lease.
The overall figure for lease liabilities in the 2021 accounts was, IIRC, around £64.3 million. That includes all leases and there's no breakdown, but unless someone can correct me I find it hard to imagine that we have any others under which the future payments are significant in relative terms. So around £64 million as at summer 2021 is your rough ballpark figure.
EDIT - I suppose we should also note that it's possible that the arrangement changed when the lease was amended to provide for increased rent in the event of expansion and for the sale of stadium naming rights.