NCBored, my setup is similar to EXPAT's, although not as extensive. Having just got my first smartphone, I had the same question you did regarding how much data I'd use and how much it would cost. Here's what I've learned so far:
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When I'm at home, playing music and video that's already stored on my iPhone is done over my own wireless home network and there's no data usage charge for that. .
Also, when I'm at home, downloading new video and music onto my iPhone is done over my cable modem which is connected to my wireless home network. I pay Comcast (my internet service provider) a flat monthly fee of $30 for unlimited data over the internet. So I don't worry about data usage or extra charges. .
When I'm away from home, I often find a free WiFi hotspot that connects me to the internet and lets me download new video and music with no data usage charge. (Free hotspots may be slow.) .
When I'm away from home and can't find a free WiFi hotspot, only then do I connect to AT&T's network and pay them for the data usage required to download new music or video. I pay AT&T $30 a month for 3GB of data usage and $10 for every 1GB over that. As a rule of thumb, downloading a standard definition movie would use about 1GB per hour and a high definition movie would use about 2GB per hour. Downloading music is a small fraction of that, as is downloading email unless it contains a video file or lots of pictures. .
That means the only time I'd really have to worry about data usage is downloading a movie when I'm on AT&T's network, away from home and away from a free WiFi hotspot. So far, I haven't been in a situation where I've needed to do that and would try to avoid it.
So the short answer to your question is "No, you probably won't incur any data usage charges using your iPhone as a digital hub for your home entertainment system." Hope the other info was helpful.