Cost effect of different component packages during PCB assembly
Personally, I'm not having any experience, but looking into same question. I tend to disagree with previous answer, as doing online quote on Chinese 7pcb, shows that 1k boards with SOIC is nearly 40% cheaper than with QFN for the same number of pads.

USD 1.3k vs 2.2k

Speaking to customer service confirms this, as they say that for QFN they have to use XRay machines to verify soldering quality, which is obviously not needed for SOIC, where visual inspection is sufficient.