Malbim: When grain is cheap, four Sa'im sell for a Sela (two Shekalim) (Kesuvos 64b). If one Se'ah sells for a Shekel, this is [famine, and it is] grounds to leave Eretz Yisrael! This shows that the famine persisted. However, since two Sa'im of barley will sell for a Shekel, the famine is not severe enough to permit leaving Eretz Yisrael.