The cost isn't in "splitting the PCB in half", it's in designing and connecting two halves. In sourcing (usually) high quality mechanical keys. Due to the small batch sizes there's also a good chance these are installed by hand, which is pretty labour intensive. Labour ain't cheap.
Labor is pretty cheap. It takes on average about 2 hours to build a keyboard. If there are no issues, it takes 1 hour. That costs about $25 total on average for labor.
The main cost is the 3d printed shells. We print PLA ourselves for cheap, but we outsource Nylon and Resin to China. These can cost anywhere from $150 to $200 for the entire thing.
The next highest cost are the PCBs. Our connectors, PCB columns for the switches, trackball PCBs, and the "brain" PCB, where all the logic processing happens, end up costing over $100-$150 total.
The total cost to build them ends up being anywhere from $250 (the absolute minimum barebones keyboard) to $400 for higher end boards.
Just some insight.
My take is that ergonomics are personal. Split keyboards work for me.
If your body is complaining, don't ignore it. Take actions. Split keyboard might be one action that helps.