They are damn near impossible to value and that came from John Hill who used to run Elite Registrations.
Your best bet is to sell it through an agency who will load the price for a customer to haggle down.
It also depends on the plate, if it has a name like Ron, Pat, Sue etc with just 3 or less numbers, it will hold its price. Best to have the letters followed by numbers. i.e Ron 123 rather than 123 Ron.
If it is a hybrid type plate contorted to look like a word or name, then don't bank on a lot.
RUS is going to appeal to Russel or someone nicknamed Rusty possibly, so a rather limited marked. It,s a well laid out plate though.
I have lost touch with the market but off the top of my head, around £2kish