Hi Dave,
charges for browsing will be the same either with option 2) and 3). But for today standards, GPRS data speed is very slow, so it will take you more time, even price is the same. Check before buying how large are pages or emails you expect to look at, and calculate, at gprs speed, how long it will take to the Zaurus to download, so you don't get disappointed.
When I say that option 2) is cheaper is because the cost of a cf-bluetooth card is cheaper than the audiovox, and in some countries you can get a mobile phone for very cheap prices; considering people usually changes phones on a yearly basis, you should consider asking someone in your family for an unused phone, of course with bluetooth and 3G at least. Also, those phones, unlocked, can easily be found in ebay.
Some more info: with the audiovox you can also do sms and voice calls; it has a headphones with micro accessory that works for voice calls, and with some audiovox it comes bundled; but I do not know of any software for doing that functionality work on the Zaurus; you'll have to know the AT-commands to send to the card to perform dialing, answering and hangup.