Depoends on state DMV rules - Ct has a "Year of Manufacture" progarm that allows for cross reference / registration to antique plates so you can run the 35 plates on the car - rules vary from state to state so check with your DMV - caution - many DMV employees are not familiar with this area so before you take "no" for an answer - ask for the DMV supervisor in hopes that they know the current rules. I've of-times had to educate the DMV clerk what form to look for!
Re value - depends on demand and how bad someone wants them - check ebay for past transactions - also there are people who trade and restore plates - could run from $50 - $100+ - they are all over the place - nice originals are in demand so don't sell to 1st offer - educate yourself first.