Price of different version of a same product

[Philippe Saugère]

Hello,
I have joomla 2.5 and virtuemart 2.
I have a question regarding the calculation of prices of different references of the same product.
I have a 5 meters roll of film  that I sell 10€
I created in each product, custom fields that correspond to other versions of the same product (10 meters and 30 meters).
In these fields, I would state the price of these variants, but without changing the original price (that of 5 meters).
I wish that when the customer chooses from the dropdown menu, only the price of the chosen roll could be indicated.
thank you

[attachment cleanup by admin]