wow $313 of that. I think i will write one for android or iPhone. 
It is basically simple.  download various tempo metronome MP3, put a timer on it, 3 minutes exercise times and rest 1 minutes. Keep playing until end of files.  connect your headset to iPhone/iPod to it and BAM. 
Estimate development time: 4-8 hours.  
Doesn't worth $313 man..