I’d just like to add my 2p in here.
I buy IPA and mix it 1:1 with deionised water, it works a treat for me and it works out around £10 for 10 litres. If it’s a particularly greasy pane of glass then I mop up with some blue roll as it’s more absorbant than a cloth and slightly more abrasive. There’s no issue of over using product as it will flash dry rather quickly and, due to the DI, it won’t leave water spots.
However, I appreciated the links above for an easier time as the majority of people would take a lifetime to get through 10L of glass cleaner.
On a separate but related matter, I have about 4L of Autobrite Direct Crystal Glass Cleaner sitting in my garage that is free to a good home if you can pickup or meet anywhere around Stoke-on-Trent.