How much does it cost to install memory for my IMAC 2012

Apple isn’t doing hardware upgrades, so you’ll need to discuss this pricing with a local Apple service provider.

iMac 27” is user-installable.

… Install memory in an iMac - Apple Support

OWC / MacSales memory has a pretty good reputation.

iMac 21.5” probably isn’t worth the cost of disassembly and component replacement or upgrade, and if you do decide to pay for it then you’ll also want to upgrade to an SSD if you don’t already have one installed; if you still have an HDD. (The HDD-to-SSD change will usually have a larger performance improvement than will adding memory. Not that adding more memory won’t also help performance of the typical 8 GB iMac.)

Least-bad update for an iMac 21.5” 8 GB / HDD iMac (short of a newer or new iMac) is an external SSD. That requires no internal access. Here is why this 8 GB / HDD configuration is slow, and with a link to migrating to an external SSD:

… Why is my hard disk drive iMac so slow? - Apple Community

Sadly because the 2012 iMac is getting on in years, you should really consider replacing it with a new Mac instead of putting any money into it.

If you are truly set on keeping it and upgrading the RAM….

The 27” has external access and is user upgradable.

The 21.5” must be dissembled and is not worth the extra expense.

see > Install memory in an iMac - Apple Support

To purchase RAM upgrades for your iMac, both OWC and Crucial are highly recommend here in the Apple Support Community.

see > https://eshop.macsales.com/shop/apple/memory/iMac

and > https://www.crucial.com/upgrades/apple/imac-systems