Reparing/replacing Fusion Drive on 2012 iMac

I have a late 2012 iMac (21.5-inch screen, 16MB RAM) that is having drive problems: SMART status in Disk Utility indicates "Failing," and I'm experiencing more frequent spinning balloons, browser cache stalls, and very long start-ups. If I'm reading System Information correctly, it's the hard drive that's the problem, not the SSD drive. I've backed up everything I need. What is the cost/complexity of replacing one or both drives and maintaining the Fusion drive set-up as is (or maybe more capacity)? Less than cost of new/equivalent (non-Retina) iMac? Can an Apple store do this? Any advice appreciated.