Kiribati (.ki) comes to mind - 900€ per year via ghandi.net or 1350€ at eurodns...

According to tld-list.com, the .th TLD is the most expensive ccTLD at $5000 and .ru is the cheapest at $2.99

Top-level .th (Thailand) is very expensive, but .co.th is reasonable ($50/yr).

But you do need to have a registered business in Thailand with the exact name as the domain name to register the domain.That's a fact for co.th, I'm not sure about .th though, but I would assume it to be true.

The new crop of TLDs are really cheap. I bought a .download for like $2 a year or something. Not all are that cheap (I bought it as a throwaway for a project that relied on my having DNS control) but there are literally hundreds of them that are.

Only issue is I don't think there's anything preventing the price going sky-high in coming years, for most of these TLDs. For a throwaway it doesn't matter, but could really hurt if you start relying on one.

Not too many ccTLDs. AI comes to mind (Anguilla, expensive for obvious reasons), but it’s still cheaper than .CD.

Many gTLDs are expensive due to their target demographics or to dissuade bad actors (eg .auto, .bank).