But the biggest reason why a Hummer's energy use is so low is that it shares many components with other vehicles and therefore its design and development energy costs are spread across many cars.
It is not possible to do this with a specialty product like hybrid.
Thats what it comes down to. Basic laws of economics. The more of a certain product being manufactured, the lower the cost per unit. Pretty much every large car company engages in platform sharing with its vehicles. They know that producing one or two different platforms/engines on a large scale is much cheaper and efficient than producing many platforms/engines on small scales. Thus, the more hybrids produced, the lower the costs will get.
So assuming Toyota can continue expanding its hybrid vehicle selection and get people to keep buying them, costs per vehicle should drop to level rivaling traditional vehicles.