I think most assembly-houses self-source some parts, and they do it in large quantities (like 1M+ parts/year). As such, they buy at extremely good prices. They overbuy themselves, and in order to get the same good pricing next year, they keep overbuying. Some of their inventory is then sold locally, and ends up in the Shenzhen market or at unauthorized resellers.

For the quantities I buy (sub-1000 pcs typ.) the pricing is usually 0.2-2.0 times the known good parts from known (authorized) sellers.

The problem is that I can not be sure what I get, and I have to implement really extensive testing for each batch.