Here's another easy question:

The Mac Mighty Mouse is great but as everyone knows the scrolling ball gets stuck after a couple of months. Eventually after cleaning it several times it's not worth trying any more.

I can source more Mighty Mice from Amazon, but they're quite pricey.

What's the best alternative?

What I'd like is:

Wired
Scrolling ball (preferably 360 degrees scroll not just up+down)
USB keyboard Plug and play - no software required!
Obviously Mac compatible (Tiger)
Less expensive than Mighty Mice (currently priced ~£25 or so)

I'll buy four if they do all that. Don't care about the colour.

Thanks!