Just curious: how much might a top radio DJ from a major metro market (like NYC) make on the side for DJ'ing in clubs?

Sometimes personal appearances can be part of an advertising package purchased by the venue with the station, and the compensation may be tied to the contract the station has with the DJ. In other words, he may not be paid directly by the event or the venue. The venue pays the station as part of advertising buy. The station’s contract with the DJ specifies a certain number of public appearances. Popularity of the DJ drives up show ratings. Higher show ratings mean a higher negotiation salary when contract is up for renewal. It’s a win-win situation all around.