I go to ATT.com, click on wireless and the cheapest contract voice is $40/month and the cheapest data is $20/month and I'm continually informed I'm the only living human being who doesn't text, so thats another $20/month plus taxes. So thats $80/month for the absolute minimum. 300 megs of data a month? Really? I don't think you could keep up with app updates alone much less actually "do" anything with only 300 megs/month. So you're more realistically $90 for 3 gigs or $110 for 5 gigs, before taxes of course.
I only download or update apps on my home Wifi connection though, and I use Opera browser which does a lot of caching.
BTW I'm still amazed at the incredibly expensive plans in the U.S. (there are lots of threads here with comparisons, and the U.S. is among the most expensive). I pay USD 20/month for my contract (voice + SMS, and we don't pay for incoming calls, caller pays) plus USD 5 per 384 megabytes.
Edit: link to one of the previous discussions
You probably don't want to have automatic over the air app updates enabled. In fact, I'm not even sure if that's an option. Honestly, 300 MB/month would be sufficient for most months even for me, and I'm a fairly heavy user on LTE. I pay for the 2GB plan mostly to cover months where my usage peaks. I suspect that 8 months a year I use less than 300 MB/month, and I use iTunes Match, occasional video streaming, and occasional Facetime calls.
More realistically, she already has a plan with a monthly payment. Which she is not even considering at this point. All she cares is how much it will cost her right now. $100 could make a difference.