Single- vs double-sided PCB assembly costs
As a single data point in this area, I just did a comparison with a company called Bittele (who build their boards and do assembly in China).
They quoted US$3815 for the assembly of 1000 fairly simple single sided boards (components on only one side) and US$3976 for the same specs but a double sided board (components on both sides).
Therefore the cost difference (in this case) was US$161 for 1000 boards or $0.161 per board.