My informant is my cousin, a 9 year old boy born and raised in Mexico City to a half-white, half-Mexican mother and a Mexican father. He has an impressive repertoire of jokes that he knows, and impresses and cracks up the family every time he tells them, usually over the traditional Mexican mid-afternoon meal, which is the heaviest meal of the day and is typically eaten with family or friends, the same way dinner is here. He is very popular in school, probably in part because of his sense of humor as well as his natural charm.
This joke was performed over “comida” as the mid-afternoon meal is called, during an hour-long family-wide exchange of jokes. He learned this joke at school.
“Una mamá vio a su hijo gateando por el supermercado y le preguntó: Que andas haciendo Lucas? Y el niño le responde: ando buscando los precios más bajos.”
Transliteration: “A mom saw her son crawling around the supermarket and asked him: What are you doing Lucas? And the boy responds: I’m looking for the lowest prices.”
I have another take on this joke from my informant’s father, who says he heard the joke in a more regionalist sense, where the mom was replaced by a person from his hometown of Hermosillo and the son was replaced by a person from the rival town of Nabojoa. I think it makes sense that the younger boy knew this version because of what it has to do with being young and misunderstanding things.