You can relax a little bit here, Guardian glass is very good.
But Guardian is a company name and not a particular product name so to compare a company to a brand is not possible.
You need to find out the actual type of Guardian glass they used.

Once you have the actual costs depends on where they bought it and what kind of unit warranty they offer etc.

If it's just the Guardian low E glass product then the actual raw glass sheets are similar.
The cost will depend on who and how the glass was made into double glazed units.
It would be almost impossible to prove which would cost more or less as it all depends on the manufacturer / supplier.

So the good news is that the glass you have installed is of decent quality so you dont need to worry and is probably a like for like product to K glass but the bad news is that Low E glass (either K glass and Guardian) is by no means the best on the market.

Hope this helps.
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