They are worth their face value at WDW, DL and on the cruises. You might be able to get more for them if you were to sell them on eBay, but unless you have $4,500,000 in "ancient" Disney Dollars, you're not going to get $5mil for them.
If you want to make some money off of them you might try asking on the collector's board to see if there is a market out there. My guess would be that they aren't going to be worth much more than face value, but I could be wrong.