It depends heavily on height, access and whether it is still producing DC current despite being turned off. As no one else has come up with a total job price, I will say £100 every six months to maintain maximum efficiency.
Why not take a before and after pic and put it on your web page where you claim to be professional and highly trained.
Steve, are you highly trained?
No mate, I'm not highly trained. I have no training at all actually as there is no training course available. Hang on, that gives me an idea.... 
That's good then - I can put some pics on my website too then as we've both had the same training 
Yes, and with the training you have had, you ae uniquely qualified to run training courses in solar panel cleaning..... Just like everyone else.
If you think we are on a level playing field, email me and tell me know what you could bring to the table for this proposed course. I will be checking my inbox...