I'm going to politely ignore your U.S. / Canada requirement and encourage you to check out TinySine. I just had them build up a batch of 100 boards for me last month -- all 0402, three QFNs, 50 parts per board, with both SMD and TH. They charged me $14/unit, turnkey. PCB + BOM + Assembly. No joke.
Once they got the boards built, they asked me if I would send them hex files to program the MCUs with to test them. This particular board had two different MCUs (one PIC, one Xtensa architecture), and had some RF components that had to be in-spec. The boards didn't have any standard programming header pin-outs, so they had to build a custom jig, and use their own programmers to write the firmware. They tested the RF performance of each board, and found two boards that were slightly out-of-spec (they labeled them for me).
Total additional cost for production programming and testing? $0.
I used to go through Screaming Circuits and Sierra Circuits, but I got tired of paying $150 / board for 50 units, and having 100 emails pop up in my inbox with constant nagging about irrelevant, self-evident, totally obvious stuff.
This is why we'll all be bowing to our Chinese overlords in 20 years.
EDIT: If you really want to stay in the U.S., check out
CircuitHub. They're the new kids on the block, and they have an interesting, fully-automated self-service system to help drive down their costs.