I've tried some cheap drybags and waterproof duffels. As far as I can tell, the cheaper manufacturers have perfected the art of making them look identical to more expensive bags, and they often function identically, too....until the zipper fails or a seam rips far from home at the worst possible moment. When, two years ago, I had to pay an astronomical price for a replacement duffel (following a long search in a strange city) midway through a trip, I swore off the cheaper stuff.
It's one of the basic questions in gear purchases: do you pay far more than seems reasonable for the incremental increase in quality offered by major brands, or do you choose instead the off-branded imitators for less money than even seems possible....with the attendant risk of failure in use? I've discovered I'm far happier paying extra initially for the extra bit of quality. Everyone looks for their own balance point on the continuum. Often, choices depend on whether you're preparing for weekend-long trips here and there, where consequences are brief and tend not to be too serious, or for months on end touring the world at large.
This ignores the obvious fact that *sometimes* you pay top dollar for branded merchandise which is demonstrably inferior--in design, materials, or workmanship--to the off-brand imitators. So far, I haven't noticed this happening in waterproof bags and baggage, but I'm sure it's out there.