Matt Mullenweg actually registered the domain for $500 per year for the first 2 years :)

.tt domains are quite expensive in general so I think he got a pretty good deal for ma.tt

https://ma.tt/2008/01/on-matt/

Interesting conincidence, the .blog TLD used by diff.blog, is owned by Automattic, the company started by Matt Mullenweg :)

Ahh. Another coincidence. It's 18 years today since the first wordpres release was made by Matt Mullenweg and Mike Little.

Google's featured snippet answer suggests that the owner of that domain has a net worth of $400mm USD. It's indeed a nice flex.

I had a cheap five-byte domain once, but the registrar decided one day to up my renewal fee to five figures, so I don't any longer.

According to nic.tt it ranges from 600 to 2000$ for 3 years. If the domain was bought on aftermarket, it was probably way more expansive though