Didn't really understand your routing or airlines getting from Cancun to US and into BZE. Could you elaborate some for us novices PLS.
Not sure it this will help, but this is what I am doing in August...
Chicago to Cancun: $332 R/T (American)
Then taking bus from Cancun to Playa (stay overnight), then bus from Playa to Chetumal, then another bus from Chetumal to Corozal. The buses and the overnight in Playa will cost me about $75-80 US + a day each way, but it works for me since I am spending most of my visit in Corozal--coming to SP only for a few days. It still beats the $850 I spent on airfare ALONE on my last trip.
(I have talked to some people who have done this, but taken the water taxi from Chetumal to San Pedro)