Where inside the Mexico City airport did you get your visa? Did it require extra time for a layover there to take care of that, or was is a simple process? Thanks.
I don't think you need a "visa", but a "tourist card". In Canada, all the airlines flying to Cuba include it in the cost of the airfare. I have no idea regarding flying out of Mexico whether it is included, or purchased separately from airline or Mexican authorities.
I suggest you go to the Cuba forum in tripadvisors which has a well-developed discussion on this subject. You could also call your airline customer service and ask them.
And if you are a US citizen it is still illegal to travel to Cuba except for approved programs.
It's important to note that the news report about the guy trying to get arrested for going to Cuba was dated 2009. The situation is far different today.
I don't know where the OP is located, but a flight from Canada may be an easier option (based on the comments I saw on Rick's Blog).