I haven't personally streamed video on the road out from my broadband card, but let me try to contribute anyhow. Given the situation of just getting a fish or learning how to fish (biblical metaphor, here) I find the second worthwhile. On and off the road I do listen to streaming radio while website windows, e.g. SPC mesoanalysis and WeatherTAP, are auto-updating and stuff. The reason I do this is both for the content of the stream, but also to monitor that the connection is up. It's not fun to look at a radar image of a storm you think is current, and discover your connection dropped and it's actually 20 minutes old. Anyway, that sort of activity for four hours per day burns on the order of 5Gb per month, or doing the arithmetic, on the order of 10kb per second. I have a legacy unlimited 3G Verizon contract so don't worry about blowing past 5Gb, which I do some months.
Well, back to your question. How much data allocation you use depends on the data rate of the stream and other usage, and how long you do it. This can be estimated by benchmarking yourself -- setting up the configuration you'd be using including a window where you're receiving a video stream of the sort you're looking to broadcast. There are many widgets and apps that will keep track of how much data is passing over the line; and you can do this with a regular internet connection. It's a good idea to practice this anyway IMHO so you don't have to futz with these things on the road as much.
So the answer depends very much on how you use the connection, but it's very possible to blow past the 5Gb line in an active week or two on the road. And that's with 3G that limits the potential damage. As 4G gets more widely deployed, the capability exists to burn your data allocation at an astounding rate AFAIK. A few years ago my wife was on a trip with the aircard and streamed a bunch of Netflix movies. That month the usage was over 20Gb(!).
If you can, do get an aircard with an external antenna port and the mag-mount roof antenna to fit. That gives you a 3-5dB gain at modest cost plus the flexibility to locate the broadband device where it's protected from heat and mechanical damage. Hope this helps!