If I recall right kumu also refers to "source". I wouldn't be horrifically shocked to learn that the use of Kumu for teacher originated first from the idea of a source or a wellspring of knowledge in the western sense. With that in mind you can see 'kumu ku'ai' as referring to the source or origination of your purchase. Maybe I'd try describing it as price being where your purchase starts?
Maybe because it's poor English, and so doesn't translate the idea most accurately. In English, a price isn't expensive - a price doesn't cost anything! It's the asking point that begins (maybe) a transaction. So the asking point (price) may be high or low. Expensive refers to cost/value of an item.