Calculating Membership Fee from a field in another table?

Hi, I've made a membership database for an imaginary leisure centre as part of my A Level coursework - only after more or less finishing my project, I've realised that I haven't provided a way for the end users to calculate fees for members.
I really have no idea how to do this, I suppose the calculation I would have to do is multiply the Length of Membership (days) field on the Membership Opportunities by Cost per Month on the MembershipTypes table.
How do I do this?! The problem really is that the cost is in months, and the length of membership is based on days, and I cant change the Length of field because the renewal date field relies on it.
These are the relevant tables and I've also attached my database (the password is "password" for any of the users):

http://i.imgur.com/DivGgOc.png
http://i.imgur.com/InpjikL.png

Attached Files

database.zip (1.63 MB)