Add variable to purchasable variant to modify price?
I think Multi Add will be the simplest approach here - it really won't be hard with that, and Luke repeated recently that multi-add will become core, so apart from some possible minor template modification later, it seems a sensible way to build this - and will be simpler (going forward as  well) than writing a new plugin for it, where you'll have to take a lot more care with your extra logic for shipping/tax etc.  Multi add just adds multiple products, but leverages core ine very other way, so it's a very robust solution.