Cans of pre-mixed drink? Gin and tonic, cocktail, even classy soft drink e.g. Belvoir farm raspberry lemonade in can or bottle of fentimans, whatever they might drink. Reasonably bulky for £1.50-£2ish.
Nice little packs of chocolates e.g. The taster packs from hotel chocolat are three for £10, I think. Those giant bourbons/custard creams that you get in Costa are 85p from Tesco bakery - I've always fancied trying one!
Little miniatures of nice shampoos, conditioners etc from Boots or similar for if they go away for the weekend.
Clinique tend to do Xmas gift packs of mini chubby sticks that would split up well. My DD bought me a four pack last year (think it cost approx £20) and I've just about used up three of them, just wearing to work on lazy days as an easy option. The fourth colour is no good for me but with five to buy for you'd probably find something suited everyone if you split a pack. I would also be barefaced on the Clinique counter about what I was doing and ask for samples - they can be incredibly generous. Last time I bought two chubby sticks for a present I got six decent samples added which really bulked it out. Also save any samples you get with regular purchases, magazines etc.
Some of the cheap nail polishes can be really good e.g. I bought maybelline superstay 7 nail polish which looks like a gel after reviews on here and was pretty impressed. It's rrp is only £4.49 and its frequently on 3 for 2. Fab range of colours so really fun to pick for their personalities/styles and see if they think you got it right.
I'd browse around paper chase for stationery bits, again matching choices to each DD.
I think if you don't buy a few cheaper bits and pieces to add bulk it's going to be hard to do a whole stocking on £50.