That's not always the norm but a lot of times they're competitive for price/performance
One of the most annoying myths of tech forums is price/performance as a redeeming factor for AMD lackluster products. Let me tell you once and for all: It isn't, not anymore.
There are two kinds of cheap sellers.
One is the producer whom structures his entire operation around lower costs. This means no-frills and often lower quality products, but as their cost structure is so lean they can offer sizable advantages in prices. Think about Toyota when arriving in the US, or Samsung TVs in the 70/80's or Southwest Airlines, or Walmart... you get the picture. AMD until Athlon 64 was one of those companies.
The other is the producer who hasn't a compelling product so they must eat into their margins in order to be able to sell the product. Examples would be GM pre-bankruptcy, Nokia, and AMD after Athlon 64.
AMD didn't design 315mm^2 chips to sell for 100USD, much less 246^2mm chips for 50. Every time you see a Bulldozer/Trinity deal, it isn't the result of a conscious effort to bring to the market a cheaper product (like Brazos), but AMD screaming "hey, we know our competitor's product is better, but we'll take a lot less profits to make our product salable".