iMac Hard drive failed - typical cost of replacement?

Seems very likely that the Fusion Drive on my late 2013 iMac has failed. Woke up to the prohibited sign, went through Internet Recovery got it as far as Question Mark in Folder. Reviewed / repaired everything in Disk Utility - which indicated there was a problem with the Fusion Drive.

Have now reinstalled Os X and the iMac works BUT:

1. It's installed Mavericks and not High Sierra and only updated security to 2016
2. Fusion Hard drive nowhere to be seen - but there is a 209.7 MB Sold State Drive which I seem to be operating off
3. all applications and docs installed via personal folder have disappeared
4. Magic Mouse does not work properly e.g. no scroll - and I can't find a fix for this

I am disabled and need help to take my imac for repair so only want to make one visit (ie not taking it around to get different quotes)
I'm trying to find out the likely cost of a replacement hard disc and removal of old and replacement by new 3TB hard drive. Ball Park figures not precise... So I can weigh up options

I've found one quotation listed by a certified Apple repairer online - but at some distance so not a brilliant option.

I was backed up - but not bang up to date so once the new hard drive is installed and I've got High Sierra back I will check where I am up to on my external hard drive and decide what to do about data recovery

Any previous experience of "what to do" and/or knowledge of what sort of ball park figure for "fixing the hard drive" would be helpful