CRC – sells in the majority to dedicated cyclists who troll forums, buy mags etc.
Halfords – sells in majority to the family cyclist who wants general gear rather than very dedicated cycle products.
Therefore Halfords are probably likely to sell more of this type of product than CRC. If you looked at Halfords as a single entity to compare directly with CRC and you could see how many of these Halfords and CRC sell in a year, I suspect that Halfords would sell many more of this item, thus they can get a better price from the supplier and so sell cheaper.
If Halfords say sold XTR cranks I suspect that the price differential would be reversed as very few Halfords cycle customers are likely to be looking for anything so high end. So in this case CRC would get the better wholesale deal and so retail it cheaper.
IMHO
No rip off involved, economies of scale.