If you want just a 1200x1200 UPVC window - doesn’t open or anything, just a plain window - I have one you can have for nowt - if you can collect from Rotherham.
Most glazing places will be happy for you to take them away instead of filling their skips.
That said glass is one of things that is still stupidly cheap, I made 3 oak frames 500x1000mm approx and triple glazed with Pilkington self clean outer sealed units cost £165 for 3. They were 52mm thick and the highest U rating possible.