You should be able to find a bit cheaper, but yes, generally you pay dearly for one-ways.
The concept behind it is that the rental company needs to pay a guy to drive the car back and fly home. Some one-ways even out, of course, but not all, as tourists have preferred directions. For example, many people drive down the East coast and end in Florida, so Florida ends up with having tons of extra cars. They often run specials where you can get a car one-way out of Florida for 49 $ for a whole week. Route 66 is the same - most people go Chicago to SF.
You should check for such specials on your way, and also consider driving the other way round (if this works for you).
Another trick I used often is to split the rental in two, for example if you spend the last 3 days in SF, you return the one-way car right away and rent another one for the remaing days (which is much cheaper as it is not one-way). You could also split into two one-ways, by 'changing' cars in the middle, etc.