Close to 0. I only use mobile data on rare occasions, to look up directions or check for an urgent email or the like. I don't see how to use 10+GB unless you're streaming video or something. I don't see any reason to use mobile data for something like podcasts. The original concept of a podcast (the name derives from "ipod" which was originally a portable audio player with an internal hard drive and no network connectivity) was that you downloaded the file from your computer onto your portable player at home, then listened to the portable on the train or whatever. I have a non-networked (non-ipod) portable player with enough good music in its local flash storage that I don't see any need to stream anything to it. I'm content to occasionally put new stuff on it at home.
I have the t-mobile $2/day plan for my ancient smartphone, which means if I use the phone any particular day, it costs $2 for that day til midnight (unlimited usage), but if I don't use it, it costs $0. So I don't use it unless getting online that minute is worth $2, which is maybe once every few months. Otherwise I use wifi if I can find it, or wait til I get home.
I use a separate dumbphone with no data plan for most voice calls and SMS. My total expenditures for both phones are less than $10 a month most of the time. Both are prepaid.