Its $375 USD but not sure if the policy has changed currently. But there is good news from Mendix, they are offering free certifications till May 31. So it's a great chance to be certified for free of cost.
To check the current price, go to (https://academyportalcloud.mendixcloud.com/index.html), choose Expert Certification, and sign in.
The price will show at the top right in the appropriate currency.
It says 400 GBP(Pound) + VAT now. Based on the payment gateway and the card used to pay the fees you can arrive at exactly how much it is.