Just go and buy a Sky Echo 2, claim the rebate, and move on...
Or alternatively, provide the certification costs for an ADS-B module... got a six or seven figure sum doing nothing?
ADS-B transmitters are required to be certified to legally operate. To get certification you have many hurdles to jump through. I spent a little time a few years back wondering what the costs would be to create an OEM module to licence, I stopped counting at £500,000. Given the number of sales is low (Relatively speaking) and must be done in assured facilities, using assured and traceable parts, it wasn’t financially viable. At a technical level it isn’t too hard, but proving to the authorities it won’t disrupt CAT and cause TCAS to go mad, is “reassuringly expensive”.
You can buy/licence modules from 3rd parties, I never got a firm price (need to enter into a commercial NDA) for a bare module. You then have to integrate that into a product, test it, and manage updating the firmware in it as and when required (which you have to pay for too).
How have uAvionix done it? Well they have had headline $10,000,000 in venture capital behind them, $5,000,000 from Airbus.