The Hoppa buses are £4.50 plus £1 booking fee - so yes, about £6.

The public buses are less. Because Colnbrook is outside London, it doesn't benefit from subsidised fares. I can't remember if it's £2.50 or £3.50 - something like that.

The hotel may also have an arrangement with a local minicab (car service) company to do airport pick-ups and drop-offs, as it's slightly awkwardly located - worth asking.