On AliExpress they seem to be around £4-6. They also have more plastic one for less than £2. I found the plastic rivet that holds the bell in always snaps on these.
Not a UK seller admittedly, and I've bought all of mine from eBay, the came from china in under a week. I can see that they're all about £4 on there now.
£7 for a rock bros bell?! I paid no more than £2 each for my three in recent years