i have a 50 cent coin from 1874

Hmmm....from your description, a case, with a serial number (bg-930 10799.64), I'm assuming that the coin has been graded by third party professional coin grading service like PCGS or such. That is a plus, for it gives a strong basis to determine a value of it without any arguement over it's condition. There are many sources to help determine value of it, the most prominent is A Guide Book of United States Coins Red Book. This book puts out an edition every year, with the 2007 edition being the 60th edition. There's also a monthly magazine called Coin World which gives the current market price and value of coins during that particular month. Biggest differences in coin values, especially with the coin you have listed here is determined by mint mark, mintage, and design variations. The PCGS website, http://www.pcgs.com/prices/frame.chtml?type=date&filename=seated_half, gives approximate values of this coin ranging from $4200 on up to $39,000, depending upon if the said coin has a Carson City mint mark (CC) and if it has arrows on both side of the date. MS stands for "Mint State", based upon a scale of 1 to 70. MS-60 is considered to be "uncirculated", and the degrees of condition above is determined in how well the coin is struck when it was minted. MS-64 is a very nice condition of this coin and would be highly sought after by collectors. To get a price that's quoted by any book, magazine, website, or collector is a matter of negotiation between you and a possible buyer. If you are interested in selling this coin, I would not recommend in taking the first offer that is given to you. It would be wise in accepting different offers, or bids, and attempt to recieve the best price for yourself.