Different ways to respond and a lot of good comments how to handle this person.
I rarely take an email inquiring seriously. If they don’t want to speak on the phone it is hard for me to take them seriously as a prospective customer. (That may not be true for other carpet cleaning companies.)
If someone calls who is not a repeat or referral I ask if the are looking for a high quality clean or are in the market for a cheap cleaning, which is most likely the case since the first and only question is how much $.
If I determine price is most important I won’t even try to determine how much square footage they actually have to clean or to explain what our service includes. I just tell them .45 per sq. ft. and $4 per step. In this case I would just say 2500 feet would cost $1,125. I can’t help but get a kick out of their reaction.

. Sometimes I tell them I haven’t had a raise in 13 years, just for fun. Afterall, they will never be my customer.
I may tell them the cleaners that charge half as much actually make more money than we do. Everyone charges what they are worth.
When they tell the cheap price by the room cleaner they eventually hire to clean their carpet about our price, it probably messes with them as well.