Guiness for whatever reason is always more expensive that other beers. That said some low ABV beers are expensive and even served in 8 or 10 oz glasses sometimes. Cost and ABV isn't always related.
I believe you need a special setup to serve Guinness correctly on tap, something about nitrogen. But that info comes from barroom banter and might not be accurate.
They have a different tap spout, and the tap handle can be pushed and pulled. But as far as I remember in my bar days we hooked it up to the same air as the other kegs.