Dan Oneata - yes, the second relation is the one I was shooting for:

> $\Phi(\texttt{true}, y) = \begin{cases} \texttt{true} &\mbox{if } 500 \le y \\\\ \texttt{false} & \mbox{otherwise.}\end{cases}$

You can buy a ticket with \$$\500\$$ or more, but not less.