Thanks for pointing out some cheapest domain extension. What about most expensive domain extensions. Are premium domains considered as most expensive domain extensions?At the moment, cheapest extensions are .top, .pw, .xyz, .eu, .ovh. But prices are not the same depending on the registar, and you will probably find a lot of discounts on domain extensions but only for the first year. Personally I use internetbs.net or OVH as registar.
Because domains .com are taken and of course some domain extensions can help in targeting specific niches better.Just curious. Why do you want to start websites with different extensions? Is it because you are targetting specific markets?
As for prices, the best thing to do is just to go to a domain registrar website. I would recommend Namecheap. Enter the name of the domain you want. Then you will get a list of available extensions together will all the prices for each.