If you have a Gas engineer, plumber, electrician, washing machine repair guy, carpet cleaner or other tradesmen, they will ALL charged a call out fee of at least 1 hours worth of work.
With some this is only the start.
Why should window cleaners be any different and charge lower rates, window cleaners offer their customers a service, and to clean the inside is an extra service which should be priced accordingly.
Don't forget that to clean the insides can cause you quite a lot of problems, you have to be careful with your footwear it might dirty the carpet and the customer might not like that.
If you break anything in the house its going to cost you, it dosn't matter what insurance cover you do or don't have because most insurances want you to pay an excess figure, this could be £50, £100 or more so it is possible that what you have broken won't be covered by your insurance cover and will instead come out of you wallet.
So remember to charge accordingly.
My mimimum is £25.00 and it goes up quite quickly if the job dictates.