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, the 1999 set is the most valuable, being the first year of the series and with a relatively small mintage, although prices have significantly decreased since the 50 State Quarters Program ended. The set in base metal, of this or any other year, is worth only a fraction as much. The silver proof sets of later years, while having some intrinsic and collector worth, are also priced far lower. The public is cautioned to research prices before buying advertised state quarter year or proof sets.