Wow that's a huge difference in fees! I wouldn't have thought RGU would be that expensive if you were from the EU! I'm from Aberdeen and RGU has a good reputation in that it has a high percentage of it's graduates being employed. It does this by doing a lot of placements for many degrees with etc, plus it only offers subjects that are employable, but I don't know anything more about the business degrees. Although being an Aberdeen University Graduate we joke that RGU isn't a 'real' university as it used to be a college until relatively recently, but that's just a bit of friendly rivalry between the unis!
I don't have detailed knowledge of Dundee Uni but I know that it certainly doesn't have a bad reputation! I'd look up Dundee more and find out it's rep for business, but for less than half the price it seems like the best option!