Ive seen the bad end of inappropiate pricing by 'canvassing companies'.
he problem being that once quoted, you are morally obliged to at least do the first visit (as this is when you will first find out if the canvassing company has under priced).
As we all know, its easier to get blood out of a stone than to up prices, (especially with a new customer base that dont know you yet), so be careful not to get dumped with cheap washes just because the canvassing company wanted the sale!
3 points that you may wish to negotiate with a canvassing company:
1), if anything is underpriced then you will get a full refund on the charge for that property (as you may have to drop it anyway)
2), tell them to avoid houses with original leaded and Georgian panes. (You are paying ofr a round to built, get them to work hard for you and to find the 'easier' houses, rather than ending up with the hard ones that no other cleaner touched for years because they are unprofitable.
3) get your canvasser to inform the customer of that the first wash may be charged at a higher rate than the regular price depending on the state of the windows / frames.
These are all important factors. You are paying someone to do this for you, so dont settle just for what he finds for you, be specific and ensure he builds it to how you want it.
Good Luck
Grant.