Unless things have changed, you can do it with most local channels (provided you are in the viewing area) for $1.50.  You can also add a superstation for $1.50.  Anything else will be listed on the DISH website and the cost varies.  You can't do this with most channels.  I called a few weeks back to add the Golf Channel a la carte (at one time you could) and had found out you could no longer do that.  It would have been $4.99 / month.