Hello,
I would suggest you to create different subscriptions plan for Pro version in order to make more attractive and convenient your service.
You can offer i.e. (the $ are only examples):
.- a yearly subscription $ 39,99
.- a 2 years subscriptions $ 35,00
.- a 3 years subscriptions $ 30,00
.- .......................................................

and/or discount for students and members of the same family.

thanks,
Luca