£300 to set up a 'not real rubbish' system?, seriously?.
I think your assumption of just how much expense is involved in cleaning windows using purified water is about the same as some of my, ahem, 'less knowledgeable', customers!.
Maybe you should have looked into what was involved before taking in the work, because for £300 i very much doubt you could even stretch to a 'rubbish' setup, even buying used!.