Yup, Shadowmoor and Eventide had these, leading to one of my favorite Commander games ever in which my friend, who had gotten cocky with his Rhys the Redeeemed deck, swung at me with like twenty elves on turn five and I was playing the new deck I built specifically to counter him and played this:
Yeah, that's mostly the same, effectively, as the card I wrote up, but it doesn't have the feel I was trying to convey.
Basically I don't want a card with two different effects based on mana spent that can be used together.
I want a card with one effect that improves based on the mana spent.
Granted that everything is Kicker, isn't that just Kicker (but with extra rules issues regarding the card's CMC)?
Kicker changes how much you pay to play the card (with the kicked effect). This one the card is always the same cost, but with the option of paying with a specific combination of mana to get a more powerful effect.