We do work for a company who replace roofs on council properties. Obviously, years of dirt end up on the walls and the windows turn black with it.
Any quote we give to them for new work is always twice the price we would charge if it was a normal clean.
If we need to jet wash the walls the charge is twice that.
The contract we have at present is for 87 2 up 2 down semi's on a housing estate and we are charging £20 per house as the scafolding is removed over the next year or 15 months. If the walls need doing - it's £40 per house.
The big big pitfall is to under price yourself. Contractors expect to be charged a higher price. We base our charges on £3 a window set i.e. a front room window may have 3 or 4 paines but it's £3 either way. Doors are £1.50. A conservatory is £10 extra regardless.
When quoting just grit your teeth and just say it. Believe me, newly built houses can be murder, with concrete and plaster on the glass, and how they look when finished is vital to the seller. Also, say it is time dependant in that if you need to spend more than x amount of time on a house, then it costs more but always be honest if you do that.
After giving a verbal quote put it in writing with a brief history of your company/experience etc and make sure the English is reasonable and get the name/address of the person who decides and send them the quote and copies to anyone else who the person you deal with says is important. Also, don't be scared to ask the person you deal with about the price they are prepared to accept. They expect you to make a reasonable profit too.
It's all about posture and belief in yourself that you are worth it.
If you can't do the work, worry about that when it happens. So far, one way or another, we've somehow managed to get it all done. I've worked Sundays before now to get the job done but then when the money comes in ........... core was it worth it or what.