The small lip of the tile above - that meets the tile below.I have seen people not clean this piece because they are frightened of getting water underneath the tiles and they seem to wash down. When the roof is finished it looks a right mess!!
Giving you an idea of price I can't do. Too many variables, access all round, particluar areas requiring cleaning, any problems with the garden area around the property.
Take into account a massive amount of tidying up, including all guttering, fascias, soffits and full UPVC, windows to be cleaned. Area all around property to be cleaned.
How are you going to get to roof level and then clean it??
Rob