Craft Commerce: fixed product variants/multiplier
I'm not sure if the base cost is global across all products, or varies between products - but either way you could use an Adjuster module or plugin to modify the price of line items in the cart.

That way you don't need to worry about creating any variants in the backend at all, and can store the price multipliers in Global fields if you want to.

To my knowledge there aren't any plugins that do this, but don't be put off my the thought of creating a plugin/module, I'm not much of a developer but have managed it myself : )

This proof-of-concept Adjuster gist should set you in the right direction, and here's another one for good measure.