How to translate high voltage 500 Vdc to 501 Vdc differential to low voltage 0 Vdc to 1 Vdc?
You can build an amplifier that lives at the 500V level and then transmit a current down to ground level. 

This avoids the extreme error and drift issues of trying to use matched resistors. To get a relatively coarse 1% error from the output means you have to have +/-20ppm (+/- 0.002%) accuracy and stability which is expensive and may be hard to achieve with a DC divider even with trimming (resistors exposed to high voltage DC tend to drift). 

For example: 







simulate this circuit &ndash; Schematic created using CircuitLab

Another option is to use an isolated amplifier designed for current sensing in VFD motor control products. You will have to add a DC-DC converter most likely to power it, and you'll have to ensure that both parts are rated for continuous operation at 500VDC input-output isolation.