Just my opinion, but I don't think people realise the actual difference between a lot of expensive items, and the cheaper knock-off versions of them. Of cause this isn't just about Turbo's, but they are of cause included.
I think people don't realise how cheap the likes of Turbo's actually are to manufacture, they aren't expensive, and they are fairly simple in the grand scheme of things.
The price of the well branded turbos mostly comes from, well, the brand. That and the fact that said brand has put a lot of money into developing and testing their turbos, increasing the price by quite a lot.
So when a cheaper company comes along and copies the branded turbos, they are essentially the same, just without the price of branding, development, ect ect.
I think the problem with some (probably a lot) of the cheaper turbos is that the quality isn't as good, but this is now going back to how simple it is to make them. So it wouldn't be hard to just take someone elses design, and still build it to a good standard.
This is most likely what a lot of companies, like XSPower do. Making their turbos very cheap due to not having to design them, but not necessarily making them crap.