Depends on whether you have a notchback, squareback or fastback, as they are all different.
Squareback pop-outs are common and can be found for under $50.
Notchback are rare as they came on only very early cars in Australia, cost around $200-250 for a pair when you can find them.
Fastback pop-outs are also uncommon as they were only standard on 1969 and 1970 Australian fasty's, they also need special seals and fiiting ( but you can now buy repro seals from the USA). Have seen them sell for $100-150/ pair.
The B-pillar ( next to the front seat backs) usually needs to be modified to fit the pop-outs, you will only find second hand glass but repro seals are being sold now.
Cheers, Pat.