It's not a joke, you're just not taking the reponse literally enough. It's trying to tell you that "What's that got to do with the price of ....?" is an idiom w

It's not a joke, you're just not taking the reponse literally enough. It's trying to tell you that "What's that got to do with the price of ....?" is an idiom which literally means "Your question is unrelated to the current topic of discussion". The idiom is used as a reponse when someone in a conversations asks something that seems like a non-sequitur. The "...price of fish?" version is the most common version I've encountered, but is probably chiefly British, but I know other variations exist (i.e. "...tea in China?", "...price of rice?").