There is no reason that they need to be incompatible. Just require that all aircraft have a functioning ADS-B transceiver and TCAS, both manned and drones. Require drones to obey resolution advisories. That will eliminate most of the midair collision that exists today, manned or unmanned.
ADS-B is not some sort of magic bullet, despite what they've told you. It *may* increase safety (though studies have shown all the glass we've put in cockpits probably does the opposite), but it's not suddenly going to prevent all collisions.
In addition, STUFF BREAKS. Your UAS that depends on ADS-B for sense-and-avoid isn't going to see that Bonanza with a transponder failure. If all the electrical systems in a manned aircraft go out, the pilot can still at least bring it down where it won't hurt anyone, the same is not true of a UAS. If you really want to be terrified, you should look at some of the FAA's results on simulating GPS system failures in a future, ADS-B only scenario.
And that's all irrelevant anyway, as there is never going to be a requirement (at least probably not in my lifetime) for manned aircraft to have an ADS-B transponder anywhere they don't already need a Mode C transponder. That will never fly, pun intended. The vast majority of private pilots will never need ADS-B out, as they don't fly where it matters. There are huge swaths of airspace you can fly in WITHOUT A RADIO, much less a transponder. This is a matter of philosophy: the airspace of the United States belongs to the people, and they should have free use of it. The FAA is only supposed to provide the minimum amount of regulation and oversight to keep everyone safe.
Finally the reason this stuff costs so much is the certification overhead (and the low production numbers). Sure, we could make it cheaper by cutting out certification requirements, but that goes back to my original statement: We'd have to accept lower safety levels. There is a legitimate argument to be made that the current certification regime may not actually result in increased safety, and that maybe it would be better for more aircraft to be equipped with SOMETHING, even if it's not certified, but that will require years of study to determine.
And as a final aside, the costs aren't quite that bad. The company I work for makes a pretty good ADS-B in/out solution for about $4k. You need a compatible transponder, too, but those aren't that bad. And a compatible display if you want TIS-B/FIS-B. Otherwise we have an ADS-B-in only solution that's battery powered and integrated w/ an iOS or Android device for about $600.