Don't know specific salaries but in my experience work is 30-50% cheaper to have done offshore on a project scale (imagine savings are better if you are paying salaries for full-time engineers). Those savings quickly evaporate if something goes wrong or takes longer than expected (very common due to communication issues), then rapidly become multiples of the original cost if you need to throw it away and have it re-written. This isn't necessarily because the offshore developers are bad, but because communicating software requirements is hard enough even without cultural/language/time barriers.
Offshore shops are a tool in a toolbox, one with a more specific use than most people think, that when deployed can be incredibly powerful but when deployed wrong can be catastrophic to a project.
As with all service relationships, building a partnership over time is the best way to get good work from vendors regardless of their physical location.
Never start with a large, mission critical project - give a small piece of work to a vendor and see how they do, and ramp up from there. Even consider giving the same small project to 1-3 vendors and see who does well - the redundancy is a sunk cost but a small price to pay for long term success.