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.
I don't think the base country will make too much difference. The biggest issues I had (that would not have occurred in the US) were with cultural barriers.
No matter the country, language can mostly be addressed with the right process and managers, but the attention and service that US customers expect is not something culturally that exists in many other places.
Getting that right, so that my team leads and PMs meet customer expectations was the hard part. I think that is where many offshore projects fail.
If your team is committed to delivering quality, and you, as the end client know how to communicate your needs and check the deliverables, things usually progress very well.
If your offshore team doesn't understand or care about this (many just don't get it, because customer service is NOT a thing in most countries outside of North America and Western Europe) then you will have frustration no matter where you go.
My clients are about 75% US based and I would be happy to talk with you directly if you have more questions or want to discuss a project. Reach out to me here: ethan [at] abovebits [dotcom]
For some clients, on call 24/7. Going to the client. Meeting their people. Handling regulatory paperwork for some projects in regulated fields. Even taking care of importing the hardware for them. Interfacing with their data people and learning their third party vendor's billing systems.
As with everything, it depends on the domain, the service, the support, and generally speaking what they're going to do for you, and which part or parts they're tackling.
Repeat business is common as when these organizations like the outcome, they want more. So that's my two cents from the vendor perspective.