two men for one day should be £150 -
£100 for the skilled guy and 50 for his helper
The window is available on ebay for £60 - So thats £210 so far
Leave the inspectors fee because thats part of the loft conversion fee
That website you used for that estimate is not accurate
Two men for one day will be dependant upon the two men. Seeing as good skilled people are unlikely to come at minimum wage a builder will want to ensure that he has trustworth professionals that can are FENSA approved.
Additionally, you will of course want to ensure that the people you use have the appropriate insurances - which of course adds to the persons inherent value.
Finally, if you buy a second hand window you will still need to be able to demonstrate to the building control officer that it meets the necessary U'Value to meet current standards, which would require testing that you have not accounted for (but would certainly be more expensive that simply buying a new window). Oh, and of course we haven't come across the issue that a FENSA approved fitter won't install a window that he does not know meets the regulations... let alone that he hasn't supplied himself (which he does so becuase he knows it conforms).
Basically. no.
You may have all the best intention for telling people how much they should be paid, or saving money by buying cheaper products but its actually false economising and you'll just end up spending more in the end.