I don't know where you're buying your toilet but there shouldn't be a $200 difference in a Toto Supreme and a similar model w/ G-Max.
Where I bought mine (online) in fact the Supreme was only $11 less than the Ultramax that does have G-Max.
I assume that you're looking for a 1-piece since you noted the Supreme.
For about $100 less than either the Supreme or the Ultramax you can get the 2 piece Toto Drake w/ G-Max. I have one and it's great plus very reasonable price about $250.