Agree that most of the tat they get in party bags is not worth spending money on especially if you buy it in the shops at 4 toys for £1 type prices. As someone else has said, on Amazon or ebay you can get 12 notebooks/bouncy balls etc etc for around £1-£2.
I like the idea of one main thing but it does mean that if any child is not that keen on that one thing they are gonna be disappointed, whereas at least with several small things there's bound to be something they like. My daughter went to a party where they all got a small but functional kite, which I thought was good.
For little ones you can do things very cheaply. I printed off some Disney colouring pictures and bought a cheap packet of crayons and split them between the bags. Also I regift some of the stuff they got from other parties :-)
My DD 8th party today, the party bags had: Loom bands (bought 1800 for £1.99 online and split them into small bags), bracelets with their names on (had the beads already, bought alphabet beads for £3), loom bracelets, stickers (12 sheets for £1.50 Amazon), glitter glue (15 for £1 in pound shop), notebooks (Amazon again), hairclips 30 for £1 in market, and a lolly (12 for £1 in pound shop). So I reckon I spent about £11 or £12 on 11 party bags and they got a really good haul of stuff they will actually use/like.