It is all about where you go and what you expect. We voluntarily went to one of the more expensive places on the island more than once (Blue Lotus) because it was just plain outstanding. But inbetween, we ofen enjoyed the roadside stands and local tacos, burritos and chicken rice and beans.
The best meal of the trip though was the invite to our friends Tim and Tina's house where they served us fresh shrimp that was just plain amazing.
So for the cheapest meal, befriend some locals and get invited over to dinner.