Most disc manufacturers (such as Ritek in case of Dynex-branded discs) test and grade media. I say most because I've seen discs are so horrible that I wonder if they went through any process that deserves to be called quality control :a Anyway, media that doesn't pass isn't always discarded or recycled because some of it may be quite useable, and can be sold at a discount, with or without branding.
When a large retailer like Best Buy decides to sell its own brand of media, it places an order with a manufacturer selected according to some criteria which are not known to us. If their goal is to make a quick, dirty buck, customers be damned, they will order a million pieces of the cheapest, crappiest media from the lowest bidder. If their goal is to create a semi-respectable in-house brand, they will go to a decent manufacturer (Ritek certainly is one) and order a higher grade. Since we don't know what Best Buy was trying to do, there is some risk involved in buying Dynex media.
It appears that I've got two good 50-packs. However, this is a very small sample and it doesn't mean all of them will be that good. But, at 18 cents a disc, it's not an unreasonable bet.