If you want Silver Halfs (not copper/nickle), you can get them for a reasonable price atCentercoin.
There is no minimum, but the government stopped making silver coins after 1988 or around then. So you can't get a 2005 pure silver coin. Some silver coins are as cheap as $2.50/coin. They have Walking liberty half-dollars(1916-1921), Franklin Half dollars (1948-1963) and Kennedy Half-dollars (1964-present). I've ordered from them, have reliable service.
You can also get newly minted 1/2 dollars (they are copper/nickle) from the U.S. Mint
directly. They come in rolls, so from the mint, you get 40 total. (www.usmint.gov
). I've bought from both.