I’ve seen even higher - around $900-$1,000 for his signature provided it is verified. I believe that’s his current fee for signings also. Considering you have his signature on the MR millennium falcon plate, the value should be even higher. The MR plate is intrinsically verified. A collector who wants a signed MR plate for their Millennium Falcon would not be able to find one easily today.
An easy way to gauge value would be to check the values of sold MR falcons on eBay. Then determine the difference between values of sold signed falcons against sold unsigned falcons. I’m guessing the plate adds at least $2,000 or more in value.