How much does it cost to add memory to iMac 2019 21.5"?

Cost is the least of your concerns. It is necessary to almost completely disassemble that model in order to replace the RAM memory. It is a complex & delicate job that is not for the casual user.

That said, it depends on how much RAM you want to add - ranging from about $55 for 16GB to $90 for 32GB from OWC.

If you want to see the procedure, there there is a video on OWC's website. Look up memory for an iMac 19,2 (the model I suspect you have.) I believe we are not allowed to post a link to it in this forum.

Hey there!

Memory is not removable by users on iMac (Retina 4K, 21.5-inch, 2019). If you’d like to upgrade the memory in one of these models, an Apple Authorized Service Provider can help. Before you schedule an appointment, confirm that the specific Apple Authorized Service Provider offers memory upgrade services.

To learn more, see: Install memory in an iMac - Apple Support.

It's also possible that your iMac doesn't need more memory, even if it's running slowly. Why are you thinking yours does?

-Jack

You may want to check that RAM is the problem before paying a repair shop a lot of money to add some. According to MacTracker, that machine came with 8, 16, or 32 GB of RAM. If you watch the color-coded Memory Pressure graph in Activity Monitor while you are running normal workloads, it might reveal that Memory Pressure is Green (i.e. that the Mac believes you have adequate RAM for the task).

A common cause of performance problems in 21.5" iMacs like yours is the use of slow 2.5" notebook hard drives, or 1 TB Fusion Drives that have a measly 24 – 32 GB of fast SSD space. 1 TB Fusion Drives originally included 128 GB of SSD space, but starting in Late 2015, Apple skimped on the size of the SSDs.

Replacing the internal drive in a 21.5" iMac is a costly exercise in paying someone to tear apart and rebuild the Mac. A much more practical approach is to get an external SSD, clone your startup drive to it, and then tell the Mac to use the external SSD as the startup drive. Your iMac has both USB-A (USB 3.0) and USB-C (USB, Thunderbolt 3) ports and thus is compatible with a wide range of modern SSDs.

Another benefit of an external SSD (besides speeding up booting and launching applications; and avoiding the need for an expensive repair shop visit) is that you may be able to take it with you to your next Mac.