I just ran into this same issue too trying out the new "all access" radio feature, burned through over a gig of my data plan in one day. With sony music unlimited I use about 500MB a week. After doing some googling I think it has to do with how they handle the streaming bit rate. There is an option to force high quality, but no option to force low quality. So you basically get the option of auto or high. The auto adjusts bit rate based on available bandwidth. The problem is that I am in an LTE area which means auto is always high(320kbps).
320kbps/8b = 40K per sec
40K*60min = 2.4M per min
2.4M*60min = 144M per hour
144M*8hrs = 1.152G per work day!
*estimated on base 10 math
So it looks like this kind of data usage is actually by design... So the question then is, how do we simulate low available bandwidth so Google music will not auto stream high quality? Dial *#*#4636*#*# from the phone dial pad and a hidden menu will come up. Select "Device Information", then in the "set preferred network type" drop down I selected "GSM only". I have now only used about 20M in the past half an hour, which would put me around 40M an hour and 320M for a full work day. With an average of 22 work days in a month, that's about 7 gigs a month. That's if I stream a full 8 hours which I would say most time I probably do about 5hrs. So maybe about 4 gigs a month... A touch higher than the 3 gigs other streaming services I have used typically go through... but way better than a gig a day! Anyway, hope this helps someone!