Did you check at the dealership? Did they verify there is no warranty coverage on it? Often times safety items like buckles have warranty beyond the bumper-to-bumper period. Worth investigating further.
OK, I suppose that is why.
$400 seems a little high. Did that quote include their diagnosis charge? My opinion would be that you should shop another Hyundai dealer in your area. You could even shop an independent shop. They can help you too. Try the Directory at the top of the screen - to find a shop near you.
There was a recent recall on this - and it's a problem with the seatbelt switch. If you've already paid for this repair, you should be able to go to provide proof (I know, 5 years later...) and get at least partially reimbursed for the repair.