How much was a Reverse of guitar of agustín pío barrios before 1998?
CREATE TABLE "banknotes" (
  "value" text,
  "color" text,
  "obverse" text,
  "reverse" text,
  "first_issued" real
);
SELECT "value" FROM "banknotes" WHERE "first_issued"<1998 AND "reverse"='guitar of agustín pío barrios';