I'm not wanting to sound rough, but there is no way the gutters should consume that kind of time for cleaning:
(1) Use a good butyl based cleaner, actually Castrol Super Clean or Purple Power both from Wal-Mart will give excellent results. Castrol in particular is very strong so start with a light mix. Both are very cost effective and available.
(2) If it's too cold it can be next to impossible to get gutters clean.
(3) I know some guys like to put their product of choice in a bucket and dip and scrub. I personally like to spray it on and kinda just watch and you will see when it's time to scrub or sometimes yo can simply spray off, just depends on the severity of the gutter and the temperature.
But, back to the issue of charging extra, to me if you want to do a top notch job the gutters have to be done. But I don't make it an extra expense, I just price the job and the gutters are included. They're not that hard to do and it is a necessity to make a complete job in my opinion.
Jeff