One can only speculate as there are only two other single-letter .com's (q.com and z.com) and there won't be others [1] . Wiki article says "GMO Internet, Inc. purchased Z.com for nearly $6.8 million from Nissan, who previously used it for the Nissan Z series cars." [2] For comparison, asking price for b.org or r.org are $500k but .org's typically command less than .com's.

[1] https://en.wikipedia.org/wiki/Single-letter_second-level_dom... [2] https://www.thedomains.com/2014/11/21/z-com-sold-for-6-8-mil...

As I mentioned in another comment [0], a domain which looks like X.com but is a Unicode X https://xn--vm8a.com/ can be bought for 9,000,000 USD and I'd think that the real X.com is worth much more, as it doesn't get converted to puny code [1].

[0]: https://news.ycombinator.com/item?id=36844079 [1]: https://wikipedia.org/wiki/Punycode