You could potentially do this with TouchOSC from an iPhone as it will send out accelerometer data. Although how accurate it would be especially on a run of show's would be another thing... Even things like Blacktrax which have had years and a development team aren't always that accurate, especially with someone as erratic and fast-moving as a dancer. Another idea which may be more suited to your budget could be timecoding cues to move the spot. As it's dance you can be much more reliant on them finding the same positions each time and although this would take more time would be infinitely more reliable and within your budget.