The 2001-S Proof Sacagawea Dollar is very common in the series. It's mintage is over 3.1 million. There are numerous other dates in the series that have mintages under 3 million. A few dates in the Proof Sacagawea Dollars also have mintages under 1 million.
Therefore, the 2001-S Proof Sacagawea Dollar can easily be obtained for a very small premium. Most examples appear to be in about PR68-PR769 Deep Cameo condition. Perfect PR70 Deep Cameo examples are a lot more difficult to find but enough examples exist that prices remain affordable to most collectors.