Talk to her. Tell her you don't want to spend that much and explain about the taxes, customs, postage, etc. She probably doesn't realise there's all that involved.
It also depends what it's for. If it's for school then, yes, it's too expensive and silly to order from the US. If it's for mountaineering, bought in the UK for fifty quid doesn't sound so bad. There are good and bad kinds of rucksacks. Best go to a proper outdoor activities shop and get advice.
Good luck.