i agree. If the RX350 were EXACTLY the same in every other way, you can talk payback only, but this is not the case. The RX400h has the equivalent of 650 lbs-ft of torque at low engine speed. This and the CVT really set it apart from its gas-only brethren.
And come on, what RX350, when driven in a mixed cycle that favors lots of city driving, will average 19.5? From what I've heard from RX330 owners, 16-17 MPG is closer to the norm. If the RX350 owners have to pump premium, the fuel factor changes considerably, since we can use regular in most areas, without issue.