I'm 100% certain a company with the resources of Behringer could make a fantastic Theremin for $99USD. But I'm 99% certain they won't because I'm 99% certain no one with any influence at Behringer actually plays the thing, and their engineers probably don't understand that optimal coils can't be bought from some coil company. But I assume they're looking to dominate the Theremini market, which isn't exactly a challenge. These things should sell like hotcakes, and it's always weird to me that making thousands of something like a musical instrument doesn't make the manufacturer pull out all the stops on the basic design. Art vs. capitalism, and capitalism always wins.
Remember the Yugo?