I estimate that it could be below $500. But that is only a rough guess, and here's why:

Most people only consider the cost of the components and labor that goes into a product when estimating what a proper retail price should be. But this is only a small fraction of the cost of making a product, especially one made in small quantities, like a decoder or even a transponder. Far greater costs are incurred from research and development, testing for compliance with government regulations, customer support, and all the other things that it takes to run a business (like rent, phones, etc.).

After all these costs are spread across a production run, then a price can be established to generate a profit for the company. Sometimes this is done on a "cost plus" basis, but if a company has a unique product with little or no competition, then it is usually best to charge "what the market will bear". That's how things that cost $5 to make end up selling for $100.

I'm still considering releasing this design as an "open-source" project, so that any individual or company would be free to use it. But first it would make financial sense to try to sell the design. This is jumping the gun a bit, as the design is not complete! I'd still love it if a competitive company would release a decoder, thus removing any need to complete the design. But that hasn't happened... yet.

My apologies for a long-winded response for such a concise question.