The law of supply and demand (which is very low) would dictate that replacement screens are very expensive. I've heard they are about $900. It's not worth it on an old iMac... better to upgrade by buying a new computer.
Better to upgrade to a regular computer with a second monitor then to avoid the expensive lesson the second time then. And better to buy the PC that costs 1/3-1/2 less than a comparible Mac.