50 State Quarters to form what is known as a "mule". Only sixteen of these specimens, produced on dollar planchets, escaped from the mint. A 2005 Minnesota double die quarter, as well as a 2005 Minnesota quarter with extra trees (another die error), have both triggered numismatic interest. An unusual die break on some 2005 Kansas quarters created a humpback bison. Relatively more common are Kansas quarters bearing the motto "IN GOD WE RUST." The United States produces proof coinage in circulating base metal and, since 1992, in separately sold sets with the dimes, quarters, and half-dollars in silver. For the silver issues,
50 State Quarters of the coins." While mintage totals of the various designs vary widely—Virginia quarters are almost 20 times as abundant as the Northern Marianas quarters—none of the regular circulating issues are rare enough to become a valuable investment. There was, however, a measure of collector interest and controversy over die errors in the Wisconsin quarter. Some designs from the Denver mint feature corn without a smaller leaf, others feature a small leaf pointing upwards, and still others have the leaf bending down. A set of all three quarters sold on eBay in February 2005 for $300 and initially saw significant increases,