i have a new imac 21.5". if the hard drive fails, can it be replaced?

If you are buying an iMac with a HDD, it can be replaced without any problem. However, if you buy an iMac with flash storage or Fusion Drive, it may be more difficult, but I don't think that there will be problems replacing them

Yes, it will be possible to replace it but I would like to be the poor tech that has to do it:

http://www.ifixit.com/Teardown/iMac+Intel+21.5-Inch+EMC+2544+Teardown/11936/1

If it's a new iMac bring it into an Apple Store or Apple Authorized Service Provider for warranty service.

https://locate.apple.com/ - Find a location near you.

My MacBook Pro 15" 2011 was running very slowly--I was spending lots of time watching beach balls. So I took it to my local Apple Store. The tech hooked it up and tested it. A big FAILED overlay appeared on the screen. Meant to scare the life out of you. The tech said I needed a new hard drive. He said that it would cost $39 for labor and $89 for the new drive, for a total of $128. That was fine with me, so we proceeded. Then he said that, instead of a 750 Gb Hdd, I could get a 1 TB Hdd for the same price. That seemed like a no brainer, but the total rose to $182. When I questioned the new price, the tech brought up the official price list on his iPad. There, the price for both the 750 Gb Hdd and the 1TB Hdd was $143. Overwhelmed, I signed up for the $182.

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$182 for a 1TB HD installed is a very good price, not sure why you be anything but thrilled?

The point is not whether or not I am thrilled to be paying $182 for a new hdd. The point is, that if I was first quoted $89 for a 750 Gb and told that I could have a 1Tb hdd for the same price, should that price not have been $89?

Did it occur to you the person may have made an error in their original quotation?

You need to discuss this with the store manager of the place you had your Mac repaired - we are all other users here and have no way of helping in this type of situation.