Newegg has the MoGo Mouse BT, which tucks away and charges inside your notebook's PC Card slot, on sale for $29.99 (plus shipping). It's normally $70.
Here's the problem with most "travel" mice: power. If they're not plugged into a USB port, they need batteries--which inevitably die at the worst possible time. Sure, there are rechargeable rodents, but then you need to remember the charging dock and/or cord. And let's not forget: A mobile mouse is one more thing taking up space in your bag, one more thing you run the risk of losing.
Enter the ingenious MoGo Mouse BT, which operates wirelessly via Bluetooth and docks/recharges inside your notebook's PC Card slot. How smart is that! Newegg.com has the MoGo Mouse BT for $29.99, plus $5.58 for shipping. That's a pretty sweet deal, considering that most vendors sell it for around $70.
Check the user reviews before you buy, however: The MoGo lacks both a scroll wheel and a middle button, and it doesn't come with a Bluetooth adapter for those notebooks that lack integrated Bluetooth. If you can live with those limitations and have a PC Card slot sitting around doing nothing, this looks like an ideal way to fill it.