In Virtumart 1.1.7 this is done by the following:
Create product
Create 2 variants (Non CMI Member, CMI Member) with their respective prices
In CMI Member variant add custom field called CMI Member Number and make it mandatory
Customer selects product, has choice of Non CMI Member or CMI Member. If customer chooses CMI Member the custom field appears and the customer cannot click next until the field is filled in.
If customer then clicks back to variant Non CMI Member, the custom field disappears and they can then click next.