Personally I used to camp at the DolGam campsite in Capel Curig, as the Pass, Ogwen, and Gwynant were just as close as each other. It's pretty cheap, plus there's a decent pub 5 minutes walk down the road.
If you really want to wild camp (cheaper still) Llyn Cwmffynnon 657564 (? - just North of Pen y Pass) is meant to be a nice spot. Really sheltered in all but the very worst weather!