General information regarding pricing can be found here: https://www.mendix.com/pricing/
But for specifics I'd contact Mendix (Sales) if I were you :)
It's per app per month € 1,875 if you go for the single app platform edition and not per user. That's is what get when I looked up the information. You could look at this url for more information, which gives additional information: https://www.mendix.com/pricing/
Also you could contact them through https://www.mendix.com/contact-us/ or just give them a call, which is shown below.
Hope this will help you.
Furthermore you can use the free Mendix sandbox enviroment up to 10 internal users!