A mintage of 31,997 has what issue price?
CREATE TABLE "land_sea_and_rail_transportation_series" (
  "year" real,
  "theme" text,
  "artist" text,
  "mintage" text,
  "issue_price" text
);
SELECT "issue_price" FROM "land_sea_and_rail_transportation_series" WHERE "mintage"='31,997';