How much does an intercooler increase gas mileage?

It doesn't. An intercooler cools and densifies the air entering the engine after being compressed (and therefore heated) by a supercharger or turbocharger. Because the air entering the engine is denser (more molecules), it needs MORE gas in order to maintain the correct air-to-fuel ratio. An intercooler is intended to increase horsepower, which means increased gas consumption... not decreased.