Dr. Greenbaum's price seems really low for a DS, I called a bunch of places and went Esquerra in Mexicali. It was 11K. So, just to ease you mind, everyone I talked to who went to Mexico, started the process with: "This Mexico idea is crazy" and so did I but in the end, it was the most rational choice out there. Everyone in my area and beyond was $30k plus. Just make sure they quoted you the DS and not the sleeve. For 15K I would have gotten my DS done here with Dr. Greenbaum. I think the hospital care would be at least 10K to be honest.
There are a couple other doctors in Mexico that appear to be good - and work mostly on Mexicans, but what I liked about Dr. Esquerra is that this board has vetted him extensively, there are lots of people here who have been been helped by him.
To be honest, my take on the whole thing is I wanted a really good doctor for the surgery, but I also wanted an awesome PCP doctor who new about the DS for the after care. That was the harder piece for me to find.
Good luck!