Ethernet is not as cheap as USB 2 for low-cost/low-performance devices. And it's not as fast (plus higher overhead) than Thunderbolt for high perfomance applications. Thunderbolt is (very loosely speaking) PCIe extended outside of the computer.
(And yes, I'm assuming bare ethernet frames, not IP or such)
We're not talking Ethernet vs USB 2.0. USB 2.0 wins in terms of cheapness.
However, Ethernet IS cheaper than USB 3.0. And Ethernet is significantly less complex. Look at carnage from all the audio interface people trying to come to grips with Thunderbolt and USB 3.0. Whereas, people implement gigabit Ethernet interfaces all the time.
And this dovetails with USB 2.0. In terms of complexity, I have found that Ethernet wins. I have started removing USB 2.0 from anything I can and replacing it with Ethernet when possible. USB 2.0 is such a pain to handle in terms of drivers while Ethernet "just works".
(And yes, I'm assuming bare ethernet frames, not IP or such)