Before I first started in 1991, a seasoned window cleaner suggested that I use 50p a window as a guide (traditional methods then).
He warned me that if I did it for any less, I might come unstuck. He also warned me that quite a few people would turn me down as the price would be too high for them.
50p seemed a bit low at first but that's because it took a while to pick up speed (I had no mentor). Also, I soon realised that leaded and georgian type windows took longer so made those windows 75p (I rounded prices up to nearest 50p).
However, after a year or two, I felt this was too low a price. The problem was that I took the 50/75p too literally without giving any consideration to access issues/frame types/minimum prices on very small properties/customers who seem to try to put obstacles in the way (both physical and mental ones).
After a few years, I realised that pretty much my entire work in one village (Handcross in Sussex) was woefully underpriced. I had hated going there for a couple of years before giving the work away (wouldn't have had the cheek to sell it). However, it was the learning curve where I made most of my mistakes and am grateful for it. I think it was around 1998 that I parted company with that village. Never had an enquiry from there since. I would work there again of course but only for proper money.