500 thousand if you know the right people
What kind of coin? What condition is it in? Does it have a mintmark?
You will first need to verify what kind of doubling it has -- there are different kinds. Machine doubling or strike doubling has very little added value, while die doubling can be worth $5.00 or more There is a good web page, with pictures, that explains the different kinds of doubling here : http://koinpro.tripod.com/Articles/OtherFormsOfDoubling.htm There is also a list of variety coins with prices here : http://koinpro.tripod.com/ErrorVarietyList.htm
You might have a flip-over double-strike which would bring several dollars from an error collector, but you should be able to see at least a trace of heads and tails on both sides. If the tails image that is on the heads side is reversed, it is possible that someone laid another coin on top of yours and hit it with a hammer. If that's what happened you just have a damaged coin. Someone familiar with minting errors would have to examine it to be sure.
Neither of these coins would be considered a rare date for their series. So, in circulated condition, their value would be based only on the silver they contain. The dime is currently worth about 45 cents, and the half is worth about $2.25