What is the approximate cost of a new hard drive in a 27” Apple computer ?

I second the answer above. I would also add that an alternative is to install an external SSD disk. It will very likely increase your speed of operations by 30% or more. Then you can install your operating system onto it and use Migration Assistant to copy over your files, etc.

There are many fine drives available. They will be cost effective, compact, easy to install with a vendor provided USB cable and many of them do not require any external power. It might have to first be reformatted, though. Recently I bought a 1TB LaCie external drive for $154. I am very happy with it.

Only an an Apple Authorized Service Provider or Visit a Genius at an Apple Store could accurately answer your question.

Depending on the internal drive configuration, the iMac may need to be completely disassembled and the labor and drive cost is probably more than the current valuation of that iMac.

Agree with what's ben said, even OWC doesn't offer Drive upgrades for the iMac Pro, I think your best bet is an external drive...

Memory Install Video (macsales.com)