I had a local glass shop cut a piece of 6mm plate glass for me. It cost me $15 Canadian, which is about $10 USD. I have been printing on it pretty regularly for over a year now. I use an electric paint stripper heat gun to heat it up to 70 or 80 ,it takes 3 or 4 minutes for the heat bed to become ready and heating switch to the hot end. I print mainly PETg on hairspray.
Shipping from the US to Canada is ridiculously expense, especially when eBay sellers use the eBay’s Global Shipping system which is a particularly big ripoff.
So buying a chunk of Pyrex is too expensive.
I think Pyrex is overkill, regular plate glass is plenty good enough given the temperatures involve.
FWIW that’s my two cents.