To some people £50 won't seem much, to others it will be a small fortune and I don't think replies along the lines of Drago's are particularly helpful.
I think I got £20 in a card for my 21st.
In your circumstances I think £50 is very generous, perhaps a small keepsake like a birthstone would be nice to go along with it. You can get them for a few quid on line
+1.
On the cash vs 'something' point, personally, I hate it when people get me 'stuff' (E.g. 'I'll get IRW a book about bikes, because he likes them'- maybe so, but not enough to spend my time reading a book I didn't need and actually have no interest in). Holiday spending money is certainly something I'd prefer over something I didn't actually have a need for.
Nowadays, I'd much rather just receive a B&Q voucher, or cash or something that I can spend on the house for example. The only exception would be for something 'personal'- what about the cash, and a nice framed photo of you and your daughter? E.g. my mum has a photo of me and my brother when we were very little sitting on some rocks in Cornwall on the wall at home- when Mrs IRW and I went to the same place on holiday this year (probably about 25 years later), I made a point of finding the exact spot and taking a photo from the same angle. We then popped it in an inexpensive frame and gave it to her for Christmas. Last time I went round, I noticed it's now on the wall next to the other one, but in a different frame

.
Is being 21 actually still significant nowadays? Pretty sure I did nothing special- certainly didn't get anything special as far as I can remember, and that was 10 years ago.
IRW