When on DL flights from Central America to Atlanta, I find you can usually get WiFi for about the last hour of the flight.

It really depends on where you first make contact with the US mainland, but coming in from Central America, it's often near the Delta of the Mississippi River / somewhere over Louisiana -- other times it can be over the Florida Keys depending on the flight path used on a particular day.

Looking at the flight tracks from Mexico City, looks like they come in over Louisiana but you may be able to get a signal depending on how close to the Texas coast you are. While they say 100 miles within a US coastline, more often than not it seems I can't get a GoGo signal via the aircraft until it's much closer to the US Coastline ... Your Mileage May Vary (YMMV) here.

When it comes to GoGo Pricing on these flights, unless you have a monthly pass or a long connecting (domestic) flight the 30 minute buy in usually does the trick.

SDF