I think it looks lovely on the model. If I wanted it as much as you do.... might pay that for something for a special occasion, but then expect to get a load of wear out of it. On the other hand, I think that £40, for example, is far too much for a top which hangs in a cupboard and that you might wear twice -so for me it's all about the' cost to wear ratio' really.
Some things do cost a fortune...cashmere, silk, but as I've said, I would always try to find something like that in a sale or buy it as something classic, knowing that it wouldn't date, would earn lots of compliments, make me feel good each wearing and thus make me feel that it was money very well spent. Not sure there are fixed rules, though. Just do what makes you happy!