For big jobs we charge per meter.
We did a nursing home last year and counted up 290 meters of guttering
We charged £350 i think only took a few hours, Luckily, it could have been very very messy and taken all day! We used a gutter vac and ladders on extra blocked up bits guiding the end of the vac to suck it all up.
Gutter cleaning with a GV is soooooo much easier. It helps if there is 2 people as well, one to "suck" and one just sort wires, hose and machine following the "sucker"
I would say price slightly heavy if you think you can get away with it as you never know what your gonna get!