I think it really depends on the type of commercial work you are pricing.
Shops? Tight as a ducks proverbial, often very, very tightly priced, great when you have lots of them but hard to get a toe hold as prices so tight to compete with.
Offices up to medium size, the sort of thing you could do WFP in an hour or so can be lucrative, I'd say to price much as you would for residential work, and then add 20% but if you price say a normal semi at £7 or £8.00 or less then - pro rata - you will be way under priced.
You should be able to work AT A £1.00 PER MINUTE WORKED as a minimum on work like this, please note that I say 'per minute worked' this doesn't mean close to a £500 day!!!
Even on residential work your rate per minute worked should be around a £1.00 a minute, which if you are an experienced window cleaner should be easy enough to go for (Average house say, 10/12 windows and a charge of £10 or £12 and time to clean approx 10 or `12 minutes (WFP that is))
But on small to medium sized commercial work, you should be able to earn at least 20% more than that.
When you get onto the bigger work, stuff that will take days rather than a couple of hours then the price will drop alarmingly on a pro rata basis.
But of course as you may be 2 or 3 days on it, your day rate shouldn't be that much different in the end.
If you are doing £250+ a day with a mixture of several small commercial accounts, each earning you the pro rata rate of £60 an hour, then you will equal that on a large job even though the pro rata rate is more like £30 an hour.
The danger is in under pricing yourself on the medium sized commercial stuff, a facilities manager isn't going to be too worried paying out £60 or £70 and whether you take 30 minutes or 60 minutes, no one is going to be noticing that.
But of course on work that is several hundred quid they pay an awful lot more attention!!
Ian