That thing that plugs into a computer is also a cellular connection and also requires additional charges.
There is no free lunch when it comes to internet on the road other than hooking up to someone's free WiFi.
Most phones can be tethered to your computer and you just leave the phone plugged in so it doesn't kill your battery. In most cases you can use up to 2GB/month for about $20 in that manner. If you go with a separate service where the phone is a hotspot or you want to use a cellular data card (as described at the start) you will be looking at service starting around $50/month for 5GB of service but there are other plans available as well.
Let us know more about what you would like to be able to do.