In reply to arakis74:
One point to note is that you will need a variety of lengths on a trad. route, say 3 x 10 cm 5 x 18cm, 2x 25 cm 5x longer extendables. So I don't advise buying a pack of 10 x 10cm.
You cam buy helium krabs £6 each then buy your slings seperately. Or go for Prowire Krabs if they are that much cheaper.
I did a bit of arithmetic a while back and found that krabs were the cheapest way of reducing weight. By that I mean that £ for £ the extra money spent on buying lighter krabs was more efficient than spending extra money to buy a lighter tent or sleeping bag or jacket or whatever.