How much to charge to wash a deck older treated wood 20ftx15ft

Really no one can tell you unless we have more information. A lot depends on many factors. For instance if your going to use chemicals or not. Do you plan on going back and treating the wood after the wash? Do you own or are you renting the pressure washer? These are just a few of many factors.

Have you cleaned decks before with a pressure washer, you can ruin a deck in a matter of minutes if you don't do it properly.

And agreed, are you using cleaning and/or beaching chemicals, will you be brushing them on first or after the first wash.

I guess the issue is, are you gong to do a 15 min quick wash a deck off, or will it be a 5 to 8 hour deck cleaning and restore