The Grey Sheet lists prices for all major U.S. coins and mint sets, in a wide range of grades, and with two price points, the "bid" and the "ask" price. The "bid" price is the price dealers are paying for the coin, and the "ask" price is the asking, or selling price.
Although the Grey Sheet can be a helpful reference when buying or selling coins, keep in mind that these prices are wholesale prices and often imply multiple coin sales with little or no customer service involved. If you have inherited a coin collection, and need to have it appraised and graded before selling it, you should also get the Red Book which will help you properly identify, grade, and value your coins.
The Grey Sheet publisher sells the newsletter by subscription, but you can also buy a "sample pack" for about $14 that includes one of each of the latest issues in the 7 issue cycle. I highly recommend buying this sample pack about twice a year for most collectors, just to have as a reference. I've gotten many dealers' prices down just by having a copy peeking out of my satchel at coin shows and stating that I'd be back later after I look up the coin in the Grey Sheet.