Hmmm. I know that the regular long step is used for almost all the cardio. I actually cannot remember how much the high step is used - I want to say not much if at all, but I could be wrong. I think it's used for sitting on during weight work, so you could maybe use a chair instead, and I think it's also used sometimes when she takes the top off (making it shorter) and uses that part for cardio, but in that case you can just use the end of your long step. Sorry, I wish I could remember the routines better. In the equipment list it does say that a high step (single step) is needed. You could check out the previews of the 3 workouts that it's made from (I think Muscle Endurance, IMAX 2 and Cardio and Weights) and see how much the high step is used.
Before I "invested" in the official-looking high step (I had to buy the little top and I think 1 more riser, because the 4 square risers on the regular long step are used to stack up the high step), I bought a 14 inch plastic stool from the hardware store for about $20 and that worked for almost all the exercises that I needed a high step for.