"I saw it online for over $700, it seems like it should be cheap or easy to make. Does anyone know of something like it for cheaper or how to make one???"
There are lots of countdown timers on the market. You can go as fancy and as you'd like. Plenty of fairly inexpensive digital watches have this feature and it's very useful for circuit training as well. Simply set the timer to beep at the time you'd like the athlete to hit for each intermediate distance interval. The Hart article beeper was set to beep on pace at each 50m segment of the interval. As an example, if the overall interval target time for 200m is 32 seconds and you want each 50m section run evenly paced, then set the countdown timer (set to repeat 4x) to beep every 8 seconds. If the athlete reaches each 50m mark on pace, then the beeper will sound as they reach the mark. If the athlete is faster than the target pace they will have past the 50m mark when the beeper sounds. If they are too slow, they will not reach the 50m mark prior to the beep. This is an useful method of teaching even pacing to any track athlete, not just 400m sprinters.
Back during the 70's the old ITA pro track circuit used pacing lights set up around the track in 5-10m intervals. The lights were set to flash at world record pace as the runners ran, in race rabbit fashion. It was actually pretty cool to watch guys like Ryan, Jipcho, Wottle and Yifter chasing those lights.