This "be" is necessary for grammatically?
For this sentence, yes it is necessary. The sentence would sound very odd if you removed the word "be" from it. If you did not want to use the word "be" you can always change the sentence structure a little. Be careful if you decide to do this because it could change the meaning of the sentence. For example: How expensive is a windshield wiper? This sentence is similar to what Ellen said but it loses its sarcastic quality.