I really don't know about the US, but in Sweden there is at least an option to get paid if you are the official, on-paper, second supervisor for an extended period of time (e.g., multiple years). You would not get rich and the university won't like it (because of costs, and more importantly, required paperwork). As a data point, being second supervisor of a PhD student is valued at about 5% of a full-time job at my current university, so you could expect about 5% of your yearly salary as an official external second supervisor.
That said, much, much, much more commonly external people are asked informally to help out without any explicit agreement and without monetary compensation. These are essentially being paid in collaboration opportunities and papers, not in money.
In that sense I would ask, but expect that the answer will be 0$.