75 pounds is not a lot for a large cake like that, I live in the states so I'm sure costs are different but that first cake here would cost me about 25 pounds minimum in ingredients and the good part of a week of work.
My sister made her dd a Harry Potter book of spells cake. Was dead easy. SHe made a rectangular chocolate sponge, covered it in chocolate butter cream and then used pre coloured fondant icing to make a wand, griffindor logo and other Harry Potter bits and bobs.
My daughter is 15 in a couple of weeks and a complete Potter fanatic. I have been trying to find a site which does these sort of cakes, but can't find anything other than bog-standard things with cake toppers. Would be grateful if anyone can point me in the right direction