what is the daily rate for this car ？

What is the daily rate for this car?
= Is the daily rate (what) for this car?
= The daily rate is (what) for this car.

How much is the daily rate for this car?
= Is the daily rate (how much) for this car?
= The daily rate is (how much) for this car.

尋ね方は違いますが、両方同じことを尋ねています。
上の方は、レートについて（つまりそれがいくらか）の質問で、
下の方は、レートがはっきりといくらなのか、の質問です。