Pricing changes based on line item fields in the add to cart form I've been using commerce_custom_product to add custom line items options to products I'm selling in the shop. The shop is selling servers and if someone when adding a server to their cart wants a bigger hard drive for instance they select it in the "add to cart" form. I want to alter the price of the line item based on which option they select

e.g. 

20gb HDD $40
40 gb HDD $70

The hard drives and other things are not really properties of the product I'm buying. They are options I'm selecting for the product. This is why I looked at commerce_custom_product.

Ultimately this question comes down to How can I set the product price based on choices made in the Add to Cart form. Is commerce_custom_product the way forward with something else bolted in? I'm very happy to code if there is a hook I could use to grab the price from the referenced entity if I was to add price to the entities that get selected in the add to cart form
I think you want to stick with Variations for this.
Otherwise you can use Rules to calculate a price