Logitech G400 (v2) for the last few years, which set me back about £20. Current mice on the market are not competitive either in terms of price or performance. Having said that, there is some interesting hype about CM Storm's upcoming products (Alcor, Mizar, Spawn v2) and, more generally, the new Pixart 3310 optical sensor may have potential.
But I'm hoping my G400 doesn't die any time soon...