Before you even start to think about prices, you need to inspect the maintenance certificates for the cradle system, in particular the anchorage on the roof, and assure yourself that it's all in proper working order. A big deciding factor will be how easily the cradle moves, whether it's powered or hand cranked, and whether you need an operators certificate - like you do for cherry pickers. It's easy to price the actual cleaning - just work out how long it would take you to clean one window and multiply by the total number of windows. After that you need to know how long it takes to reposition the cradle between windows (or floors, if the cradle spans the whole length of the building). Then there's the question of getting round the corners to the other faces of the building. Is there a separate cradle for each face, or do you have to move it round? That could be a time consuming operation - try it before you commit to anything.
I reckon you should do a "dry run" in the cradle, and time yourself - that way you avoid any nasty shocks after you start work

Cheers,
Ian