I use my son's HTC Tilt 2 as a wifi router. Software was free and cost is $10.00 a month unlimited data because I have a family texting plan and he is grandfathered on the older $10 plan. If I were to sign up today, cost would be $30.00 a month unlimited data.
I'd highly recommend using a phone that can support wifi router operation and a data plan. Much cheaper than $60 a month and a 5gb cap.
I've been using a cradlepoint phs300 for the last few weeks with a sprint 595u.
I find it reliable and speedy enough. I can usually get a couple hours of battery life.
Buy.com sells it for $99 plus shipping.
CradlePoint PHS300 Personal Wi-Fi Hotspot 3G - PHS300 - Buy.com
The 3G modem connects to the Cradlepoint router, starts up the modem and distributes it via wifi. It is completely separate from the iPad, laptop, or any other wifi device you have. You still need a 3G modem service from a service provider to get this going but its just an alternate to the iPad 3G since I already have 3G service and don't want to pay for additional service.
Check the link for a bit more info: PHS300 Personal WiFi Hotspot - 3G/4G | Cradlepoint Technology
I just ordered a Cradlepoint 300. I already have the Sierra 597 USB modem (unlimited plan for North America). I plan on using it to ease my data roaming costs for my iPhone as well. Once the 3g connection is converted to WiFi, the iPhone, iPad, iPods, laptops, etc. should all be able to use it.
The reason I began this search was to determine if the extra expense for a 3G-capable iPad was necessary. As long as it performs as expected, I'll skip the 3G-capability in the iPad, and stick with WiFi.