I've cleaned a few and never had any issues. Many times you can just clean them with your house wash mix. Probably depends on the type of metal or paint but the ones I have done have been pretty easy. BE CAREFUL with run off, there are no gutters and run off will be real bad. As far as pricing goes, don't like to give prices. Every market is different and everyone's overhead is different, but it's just like pricing everything else you do. Figure out how long it will take times how much you want to make per hour.
Good luck