SBCs are not just RPis. Other brands can still be bought cheaper.

A few, until their current stocks run out. Orange Pi already increased prices (their boards are similar price or more expensive than equivalent Pi's now), and Radxa seems to just stop selling certain models (at least in NA) once they run out of stock.

Arduino has one of the cheapest 4GB boards now, but I wonder if it's just because they made a ton and the demand for their strange board has been low?