Perfumes from Dubai are indeed genuine, well, except for the cons and scammers selling knock-offs. But as a rule, they are very genuine. They are cheap when sold in the market because they have access to so many, it becomes a high competition market, and they have to sell them cheaper. Plus, by selling them more affordable, more tourists are attracted to come and buy them, which allows them to sell more.
If the perfumes were knock offs, word would have gotten around by now, and tourists would not be so intent on buying perfume there so it would certainly give the good guys a bad name. The traders have good access to great perfume at rock bottom prices so they can sell them cheap and still make a profit.