hi go along and have a look at them, as they might be all green, take a few pics if you can and put them up on here, there is no point charging say 50 a con and it is that manky that it takes a day just to get clean,, like wise on a spotless one
see if you can get easy acces as well, people where i live have a hbit of building them in the worst places possible
i do a house that has a con with 20 panes of glass, 5 down each side and 10 across, it is big, the owner was quoted 50 by some one to clean it and said no, and this included the roof
i would not give a price at the time, just take there no and phone them the day after on a few jobs i have given the price and then regretted it when doing the job as turns out to be a pain, or you spend hours on one part of roof as it wont come clean when dry, take all the info home and think about it
try using tfr as this is meant to be good a cleaning cons, i think you cover the con in it then wash it off
i would do a complete clean for the price as well