Here's my advice.
Pull up a bunch of the razor restorer websites and look at the prices of the razors that are of different restored quality. Stay with the same brand for comparison, look specifically at the brand you are looking for.
Then think about the effort/money it would take for the eBay razor to get to that level of quality. Then subtract that from the restorers price.
That is what I use for my top bid.
It's really subjective, but it's a baseline for me to stay within a limit.