This offering from KJ Global makes last year's slim mouse from Samsung look somewhat elephantine (although I'll admit that the Samsung one is cuter). The G4 model from KJ Global is just 5mm thick and, when folded, measures 94 by 53 mm.
When unfolded, it looks a bit weird, rather like something you've just made from origami during a slow work day, but it gives you two buttons and a touch wheel in 10 seconds flat. You can wind the cable inside for storage and it comes in blue, ivory, pink and silver. As for cost, I haven't got a clue.