Am I being a bit thick here, but why is there 2 different prices for it with and without the soundpack?
Because the "soundpack" is basically an additional amplifier, bolted under the passenger seat. So the amp adds an additional cost
My salesman also told me that the easy way to tell if it has the soundpack is if you have 10 buttons on the dash for the stereo. If there is only 8 then you haven't got it.
Nope, that is wrong. The number of buttons simply indicates which type of head unit is fitted, and has nothing to do with the number of speakers fitted, nor weather any "soundpack" amp has been fitted. 8 buttons on the head unit indicates the povery-spec RCD300, which is a single CD player, whereas 10 buttons is the upgraded optional RCD500, which has an integrated 6 CD changer.
Edit: Just looked again and it seems it's more expensive for a Golf S than any other model. 
As others have stated, that is because the Golf S doesn't have any rear speakers at all as standard - so when upgrating to the RCD500, instead of just bolting in one extra speaker (per side) in the rear doors, they need to add three extra per side - expensive!
