Why do different TLD have different prices. What or who sets it?

The registry sets their own prices. For the new generic TLDs it is basically the Wild West - they can charge whatever they choose, hence the fact that most new gTLD registries go for bargain basement pricing in the hope that they can get enough volume for someone to want to buy them before the whole sorry mess comes tumbling down.

For country level TLDs it’s a little more nuanced I guess. Extensions like .fm, .io, .co and .ai have either managed to position themselves as a fashionable alternative to the very crowded .com (or whatever you first choice is) or have been fortunate enough to have an extension that “fits” a particular industry. Because they work and for some people have become an “accepted” alternative to .com they can charge a premium for brand name fit.

https://www.namecheap.com/blog/why-some-domains-cost-more/