JFZ - Good question, since the B has the equivalent of an extra engine. However, the official line is that both are about the same price.
And before anyone starts going "C model for the RAF, greater range" let me repeat that the C is very heavy, that any range improvement is largely attributable to replacing the internal gun with JP-8, and that the C will be slower than the A.
So if the RAF wants range, it would be a lot cheaper to pull the gun out of the A than to try to lighten the C.