Hi, how can create a different price depending on a previous selection ?

I think this guide might help you to achieve your scenario.

https://www.jotform.com/help/316-How-to-Show-or-Hide-Fields-Base-on-User-s-Answer

It works based on conditions, here is another guide to the general use of conditionals logic

https://www.jotform.com/help/57-Smart-Forms-Using-Conditional-Logic