I don't know for this particular case but usually expensive stuff has expensive components too, especially in low quantity

I have a B210 but the project indicated that it was only compatible with the X310 with two daughterboards.

From Hardware Requirements > SDR: "To sniff only downlink traffic from the base station, one can operate LTESniffer with USRP B210 which is connected to PC via a USB 3.0 port. Similarly, USRB B210 should be equipped with GPSDO and two RX antennas to decode downlink messages in transmission modes 3 and 4."

That’s one of the lessons learned when hacking your own thing. After enough projects and buying the parts & pieces (you never buy just one component), you end up with a stash that eventually means you don’t have to buy anything for a future project. It’s part of the cost of being a hacker.

From rather ancient memory, the high-end Ettus stuff is much, much more expensive than its BOM cost. You're paying for a well-designed and well-tested product with excellent RF performance and a good ecosystem. You buy it and it works.

You could design your own board(s) with the same parts, and you could build it for much less money. But you'd need to know what you're doing, and the design stage would be expensive.