A 20-peso banknote issued by Banco de México is worth exactly 20 Mexican pesos (MXN) in face value. Its purchasing power in Mexico is whatever 20 pesos buys locally (e.g., a bus ride, a small snack, a newspaper).
As of February 2023, the exchange rate hovered near 20.50 MXN = 1 USD.
Critical Caveat: Exchange rates change daily. As of mid-2024, the rate is closer to 17–18 MXN/USD, making the bill worth ~$1.10–$1.18. Always check a live converter (XE, OANDA, bank rate) for the current value.
Standard circulating 20-peso notes (polymer G-series, issued 2007–present) have no numismatic premium. Only rare errors, early series (pre-1993 "nuevos pesos"), or special commemorative issues might exceed face value to collectors.