My standard helix was done with a gun at Claires and I was lucky with it because I didn't have issues with it but many people do and there is a chance of it damaging the cartilage and you will care if that does happen. Healing time ranges but usually a helix piercing will take months and months to heal, from between roughly 6 months and a year or so, about 4 months minimum. If you get it done with a gun though the healing time can be longer than if you get it done with a needle by a proper piercer.
Price will depend on the stud you choose.
Personally I'd just go to a piercing/tattoo place, they are proper piercers (completely trained), they are cleaner (Claires do them on the shop floor with loads of people around, it's hardly a clean place to pierce somebody), they use better quality metal (often titanium) which has a low chance of causing irritation, they always use the needle method which is the only method which should be used and they actually know what they are doing. Claires staff watch a video for their training! Also in Claires the metals they use aren't good for piercing with, they all have the same metal backs and they use studs with butterfly backs which increases the chances of it getting infected.
I actually paid less to get a piercing at a proper piercing place with a good rep than I did for any of mine done at Claires! Personally I'd never let somebody from Claires touch my ears again.
If you do go to Claires and they tell you to turn it, don't turn it at all! If you turn it you will pro-long the healing and will just irritate it. It should be left alone and not touched unless you are cleaning it.