Not sure what I actually spend on ds and dd, but I probably buy more things for dd, because she has skirts and dresses, whilst ds only has trousers (obviously tops and other things too, but they probably work out about the same). The fact that dd is both very appreciative of clothes, and enjoys shopping does make a difference, and I don't feel bad about that. ds gets more books (well, when I say ds - he thinks they are for him, but really they are for me ).
I usually make a list before shopping, in terms of what they need, and then if there is something really nice I see for either of them, that goes in the bag too. On the whole I am more likely to see nice things for dd, because there are in general more nice things for girls than boys.
The biggest struggle I have is finding appropriate things that fit. dd at coming up 5 is in size 7-8, and ds (6 1/2) is so skinny that getting clothes long enough than don't fall off is a real struggle.
The last big shop I did was at Pumpkin Patch for ds, and at Mothercare for dd.
Oh yes, and the other thing that makes a huge difference is school uniform. Once his uniform is bought ds can get by quite easily on two trousers and 3 or 4 tops all term. Generally school uniform is much cheaper than "real" clothes.