I take it that this is part of the normal banter, and you wanted to address it without being excessively confrontational or causing potential awkwardness, but also wanted to make sure the concept was shot down, in case there was a grain of hope/expectation behind it.
The way to do so is to shoot it down in a semi-joking manner. In my lifetime of experience where the normal dynamic of communication with my friends and family is sarcasm and quips, it's a way that can be taken both as a joking response if that's truly the spirit in which the original comment was offered, but in no way can be confused with acceptance of the premise. The most obvious one that pops into mind for me would be something along these lines -
I'm not that cheap. If Bob wants to see "the goods," it's going to cost him his car, not just an errand.