FCs are so darn expensive man think I could just wire up a jumper from the esc input wires to the FC to get voltage and power the rest of the OSD?
depends what you want in your OSD... if you only want battery voltage, then you could probaby get by with something less expensive, but if you want orentation/etc, you need an IMU and a microcontroller to process the info and send it to the OSD (and a voltage regulator for the parts). At that point you have most of the components that make up a FC (microcontroller, IMU, OSD chip, voltage sensor, voltage regulator, various discrete components), only think missing is a current sensor... (and those are fairly cheap). About the only way you could get it 'cheaper' would be if you used a cheaper microcontroller, and designed the thing yourself (and that might not end up cheaper as you would be purchasing parts retail instead of wholesale).
If all you want is voltage, just get a receiver that has a voltage sensor on it and setup telemetry and have your radio read out the voltage for you.