We have one you can have for free. It is from an old glass dining room table we used to have. Rectangle, maybe 48″ by 36″. editing to add that if you’re interested you can email me at fatkitty at comcast dot net
Cost is going to depend on thickness, size, type of finished edge, etc. It’s best to call a place and talk to someone. The owner at White Center Glass was very nice and helpful. I just needed a piece to place inside a tray that my FIL made for me. Was something like 13×20. Recall it being $9 or $10.