How do I make an equation to subtract different between an included product and upgrade to a different product?

I have a Product Bundle that someone can order.

The Bundle is an Object, and is connected to a Product Object. So, the Bundle includes 1 product. I want to a customer to be able to 'Upgrade' to a higher in end product in the same Product Object on the order form.

And i need a way to show the price difference if they want to upgrade.

So, for example, a customer selects a bundle called Internet + TV Bundle. The Bundle has a connection to an Object called "Internet Speed" . The default speed for this Bundle is 10Megx3Meg for $75.

I want to give my customer an option to upgrade that Internet Speed to, say, 20Megx5Meg. The cost of the 20Megx5Meg is $106. I want to show the upgrade option in the dropdown as "20Megx5Meg - $31"

To get that $31, it needs to be a formula that takes the Internet Speed that is originally included in the Bundle, and the 'Upgrade' speed subtracts from that. What connections would I need to make to get this to work? And the correct formula?

I am thinking that once I get the formula worked out, I will just make a Text formula to show the Product Name - "formulatotal".

So, in total, there are 3 Objects. A Bundle Object that holds the Bundles. An Internet Speed Object that holds all of the available Internet Speeds, and an Order Object that gets all of the orders placed.

Thanks for any help!