I charge £35 per side that's for vacuuming out all of the rubbish and cleaning the soffits, fascias and outside of the guttering, not had any problems so far.
What I do is talk them around and explain how UPVC is is a product from the petro-chemical industry, and how exhaust emissions affect them discolouring them etc. which then allows funghi and algea purchase allowing it to grow and prolliferate, if it's not cleaned annually then they will become damaged beyond cleaning and need to be replaced.
Like I say it works for me!