If you already have a car GPS and are OK with using it, cheapest solution is get an external battery pack with USB output. I've seen rechargeable ones, versions with solar panels (SparkFun has a 3500 mAh pack with panel, albeit undersized for the battery capacity), and packs that use AA or AAA batteries. If you are even minimally handy with a soldering iron (or have a friend with the skill), build a MintyBoost kit (AdaFruit and I think SparkFun) - recharges USB devices from 2 AA batteries in a small size Altoids can form factor. Or buy one of several now-available dynamo systems designed to power and recharge USB devices on the bike.