Very cheap. About 15 bucks. I tested it again this morning, and got 6 full presses on the brake, and one more that was effective but not as powerful as the first 6.
I see it as just an added margin of safety, because I practically never press the brake that many times. On a big hill where I didn't want to grav to 60mph though, it might pay off.
Oh, I'll have pictures this evening. I'll try to remember to take some when I get home. It's really pretty much exactly what you'd expect though. A PVC pipe, capped on both ends, zip-tied under the hood with a hose connecting it to the brake booster hose. :P