Today it's actually not outrageously expensive to do a multiple microphone array. REW has a $100 pro option that functions as a software multiplexer. Inexpensive calibrated microphones are under $100 and you can find a 4-channel USB interface for well under $400. That said, the benefit is time saved (though now you have to worry about SPL calibrating the microphones) rather than measurement precision or accuracy compared to moving a single microphone or doing a moving-mic measurement.
