I paid $1.80 for my first domain (which was a .lol TLD, a little obscure but worked for me). It’s now renewing at $26, though. The more well known ones are a fair bit more expensive though, and rarely have introductory sales. There are a lot of random weird domains nobody’s ever heard of that only renew for a few bucks a year on going though.

It wouldn’t work for a Lemmy instance, unless you don’t mind changing instances every year, but you could theoretically get domains for less than a dollar for at least a few decades by constantly changing domains every time you’re due for a renewal, to make the most of the introductory discounts lots of them have