I'm confused, is it a "huge" cost, or a "non-zero" cost?
Both, because you need to multiply it by planes in service.
A few seconds on Google shows there are around 20,000 registered commercial airliners, and around 145,000 registered aircraft (including commercial aircraft, corporate jets, personal airplanes, military aircraft, and so on.) It doesn't include non-registered aircraft, of which there are many.
But the costs multiply. So when you start with $100K for the device plus installation, you are looking at $14B for the first pass. Then your small annual fee multiplies to perhaps fifty million every year in upkeep and service fees.
If you are talking about a major aircraft like a commercial B777 passenger craft, the installation and upkeep is relatively small. These massive aircraft are expensive to buy and maintain. The amortized cost per passenger over a year's flights is going to be a fraction of a cent.
When you are talking about smaller craft like the super common Cessna 172, the device is going to be about 1/4 of the cost of the airplane. All the little utility aircraft are the most common type of aircraft, even though most of us only associate airplanes with the giant cargo jets and passenger flights.
Ultimately let's assume they are looking around $14B initial investment plus $50M/year continuous cost. All of that money to get a little information once every few years when an airplane gets lost over the ocean. Is it worth it? Perhaps it is worth it for the large commercial passenger airlines, but not for all aircraft.