There is no SHOULD - charge what you feel most comfortable with, this way you'll deliver the price with confidence, and pick up more work.
1 off = bespoke, you will unlikely see this customer again, or maybe in several months at best. Would you like all your customers to be like this ?
It's not just the time on the glass, but getting there, fitting it in etc....
1 off work is 3x a regular 4 wky clean, majority of first cleans are 2x the regular clean, unlike mick I'm far too sceptical to believe it's an investment, first cleans are an intensive clean that take longer than a reg clean and are priced accordingly, however I frequently offer a loyalty bonus where the sixth clean is 1/2 price
Darran