It's a bit like a customer in the street asking you for a price on their unfamiliar property. Only a fool would rush into giving a committed price before looking at the property.
Too many variables with houses to give an accurate profitable price.
This is why so many window cleaners have low prices as they rush into giving a price on the spot so they won't lose a potential new customer.
Best advice is go and view the property and don't be rushed into giving a low price. Work out your profit price, stick to it, (don't let your fear of not obtaining the customer put you off). Inform them of your desirable price, then (most importantly) keep quiet until they reply. You can always haggle over small differences afterwards, i.e. the price, frequency, extras,etc.