How much does it cost for going to dentist in Japan?

It depends if you have the Japanese national health insurance or not. If you do, I'd say a regular check up would be around 3,000 yen. If you don't, you'll have to pay 100% upfront and it'll probably cost around 20,000 yen. But this is just a guess, so make sure you bring extra cash, as clinics in Japan don't generally take credit cards.