I've searched through all the Manuals, Tutorials, Videos and the Forum to find what should be a really obvious, basic database requirement - but I'm still stumped!

I'm building a membership database. I have members (based on the User object), a Membership Types content type (based on the Article object) and a Subscriptions content type (based on the Article object). So when a Member buys a Membership Type, this is recorded in the Subscriptions table. 

The Membership Types table contains fields for Membership Type (Article Title), Price (Text) and a few others.

The Subscriptions table needs to record: Member (JForm - User), Membership Type (Select Dynamic) and these work fine. They pull in the correct info and store it correctly. 

BUT the Subscriptions table also needs to record the Price of the chosen Membership Type. How do I get the Price field from the Membership Type table into the Subscriptions table? Once I select the Membership Type I have established a join between the two tables, but I can't find a way to use this join to pull in the related Price field.