£40 is a lot for something that's only suitable for high summer, unless you live in Dubai or somewhere
I occasionally buy the DDs expensive clothing if we have a wedding or something to go to. I am more likely to spend on dresses, as they're grown out of less quickly - you can buy them below the knee and let them steadily creep higher - and you can wear them all year round with tights and a cardie.
Now that I have two DDs I do spend a little more on DD1's clothes, knowing that her sister will be in them three years later.