How much would it cost (approximately) to upgrade an 8GB RAM to 16 or 32 GB for a 27" 2012 iMac ?

If you go to Crucial.com, their web site will tell you exactly what it will cost to do those upgrade.

An addition to Allan's reply. You will get similar reliable service from OWC, too. It really depends on your location as shipping costs, taxes and so on can impinge.

https://www.macsales.com/ Sorry, I had to edit the OWC link.