As fitting only requires unplugging two leads and unscrewing 4 screws to take out old harddrive and then replacing with new drive in - I wonder if you are also having the contents and operating system transferred from your old drive to the new. If so then £80 total for new drive, fitting and reinstall is about the going rate in local pc shops.
Make sure it is a new harddrive with warranty. If you know the make and model of harddrive, post info on here and we'll advise on the price etc.
Here's a link click here to installing an IDE hard drive, it's not a difficult process at all, turn off the power at the wall socket and back of the computer, leave it plugged in (this will keep it earthed to the house) and touch a part of the computers metal framework now and again while working, this will drain any damaging static electricity. If your drive is SATA then there's a link to installing one of those at the bottom of the page.
If you're installing an additional drive to the one you have in your system then £80 would get you a lot more storage space than 160GB if fitting it yourself. But as others have said, if the drive is a replacement for your existing operating system drive then it may be worth paying to have everything transferred from the old drive.
I'd point out that replacing a hard drive is a very, very good place to start easing yourself into doing all your own PC repairs and upgrades.
It's not a bad price if, as others have said, it includes all transfer of data, and a working computer. However, anyone doing that would, I suspect, double (triple) the size of drive for a bit of future-proofing. This drive might have been sitting on a shelf for quite some time, in my opinion.
Why not give it a go yourself? Post why you need a new drive, and get recommendations. The forum members will certainly talk you through all the steps required. Then you can spend the money on something more fun!
I noted on your previous thread that you do not have a back up installation CD I suggest you make one as per: Create your Master CDs or Master DVD. Use the program called Master CD / DVD Creator to create a back-up of your installation media
Also get to know your system and components and what to do click here Packardbell Support Imedia 1304