Someone educated me on this type of thing a while back. Chat.com - and other highly reusable URLs - is a non-deteriorating asset that has a value on OpenAI's ledger somewhere. It's not like they bought $10,000,000 worth of catering or fuel or labor even buildings. Because someday, the owning entity can sell it for about what they spent on it. And I imagine there is some kind of financial instrument to take out a loan with the domain as collateral. My point is, it's not really like they "spent" the money as much as they just parked it in a domain name.
Though you are correct it is a durable asset. (As are buildings.)
15M is like one month of salaries for 500 engineers, so like 40% of their whole eng. team
They spent more than $15.5 million, according to the article