You simply unplug the white connector under the dash.
Pictures here:
DIY: Disabling DRL (2000-2004) - Subaru Outback - Subaru Outback Forums
I used to also pull the ebrake up to disable them, but once I adjusted my ebrake properly I didn't want to grind into the brake to turn them off. Plus the brake light coming on was annoying.
Also. 60$ for .5 MPG isn't too bad. It's a shame it's only at night, or I'd jump right on it. Although, with just 70 watts saved it'd probably be more like .25 mpg. Which is a bit harder to swallow. I actually have interior and license plate LED's but only because they burned out on me and I didn't feel like changing them ever again.