Modern sewing machines are cheaper because the intricately machined cams and drive mechanisms that connected the upper mechanism that moves the needle to the lower mechanism that moves the bobbin are no longer used. They're replaced by a pair of stepper motors that are driven by a microcontroller. Instead of having collars and set screws to adjust timing, it's all solidly built and ruined the minute the stepper motor slips or a tiny plastic part breaks and something no longer actuates. They're unrepairable because you'd have to get the actual OEM somewhere in China to release the source code and put a jtag pinout on their boards rather than getting the chips loaded at the factory where the main board is made. That's the difference. Same reason that cam machines have been replaced with vertical machining centers -- CNC is just waaaay cheaper.
