Here is a report from the lunatic fringe: You may not need a data plan at all.
My wife and I both have smartphones (Nexus 4s). We happily use them as smartphones but we do not have data plans on either one. When we are at our city home or at our lake home, we have free wifi pretty much anywhere we might go including doctors, stores, and restaurants. The only "on the road" function we need is navigation, and Copilot Live provides that nicely with all maps downloaded to the phone (via wifi) and available 100% of the time.
We're on a family plan with
http://www.puretalkusa.com/ and it is very cheap with no requirement that we buy data. (The do have cheap data though.) We are light phone users as well; most months our total bill for both phones is $15. Sometimes $25.
When we travel within the US, we buy a slug of data from AT&T for one or both of our Nexus 7 tablets. The current best deal for us is 1GB for $25, useable over three months. That gets us yelp, urbanspoon, hotels, ... etc. as well as email everywhere. Neither of us have any interest in streaming media, so 1GB is more than ample for a few weeks of road trips.
Music? I have 6 or 8 gb of legal music on my (16Gig) phone. Mainly I use it to help me sleep when we're flying internationally, so a data plan wouldn't help me there anyway.
Obviously, YMMV.