Does anyone know if there is a shuttle between the airports in Toluca and Mexico City? Free or inexpensive?
No, you will have to take a taxi in to Toluca, from where there is an hourly bus to MEX
A couple of years ago Volaris had a shuttle between the two for Their customers. Do not know if it still scheduled or not.
Volaris now only has two flights a day from TLC and no shuttle any more. The Interjet shuttle does not go to MEX, but to the Santa Fe district (miles from anywhere!) and it is ONLY for their customers. I assume the OP is coming in on Spirit.