user: Question: How much was a Reverse of Guitar of Agustín Pío Barrios before 1998?
Columns: ['Value', 'Color', 'Obverse', 'Reverse', 'First issued']
Types: ['text', 'text', 'text', 'text', 'real']
Sample row: ['2.000 ₲', 'Magenta', 'Adela and Celsa Speratti', 'School parade', 2008.0]
SQL:

assistant: SELECT "Value" FROM "Table" WHERE "First issued" < 1998.0 AND "Reverse" = 'Guitar of Agustín Pío Barrios';