What's a simple, cheap treatment for my muscle spasm?

Heat or cold. A hot blanket or heating pad applied to the area of spasm will often do the trick. A hot bath or shower may also help. Drinking hot tea or warm milk may also relax you enough to end your spasm... This is all presuming that the spasm you are having is a result of a temporary overuse syndrome or a strain. If there is another cause of your spasm it may not work. If they don't work - get it checked.