Be sure you ask a provider if they do that work before hauling your iMac to a shop. This Apple article suggests that some Apple Authorized Service Providers will not attempt that difficult task:
The note at the end of the Apple support document on upgrading RAM in iMacs says it all:
https://support.apple.com/en-us/HT201191#note
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.
Cost: I did a "spot" cost survey of computer service shops in the NW US a couple of years ago. If an independent provider agrees to do the work, expect to pay for at least two hours of labor at US$70-80/hour in addition to the cost of RAM and the required reseal kit.
Jack is correct in asking about performance Entry-level 21.5-inch iMacs from 2012 through 2019 shipped with slow and under-spec mecahnical hard drvie and this cause them to be slow to start up and slow to launch apps. However, once open apps run fine. Those are classic symptoms associated with complaints we see here regarding slow 21.5-inch iMacs.
You can throw hundreds of dollars worth of RAM at that issue without make the drive problem better. If that is the case, Jack will have a better and far less expensive solution for you.