Depends what you do while surfing. When I went to some website (I think it was MacUser.co.uk the other day, my little old {Windows Mobile} phone showed it downloaded around 890 kb (that's not far short of 1 MB) just to grab the 'home page' - probably a load of photos in that count.
OK, one doesn't download the same page dozens of times, but if you use YouTube and so on, you could 'clock up' quite a bit of data. Download some MP3 music track and it may be 4 to 5 MB and if you went onto some government website and found a report that you wanted to read, before you knew it you might have downloaded 12 MB of a PDF (not every site shows how large the downloads are) - and don't always tell you that the link gets a PDF rather than just another web (HTML) page for you.
I've seen some comment that in the course of a month they download around 100 MB in total, but everyone is individual and chooses different web sites to visit, for different amounts of time and may download dramatically different amounts.
I suppose the thing to ask before signing a contract is 'how easy is it to check how much has been downloaded' and secondly, 'if I want to download more, is there some bolt-on that I can pay for'.
Someone said in a different thread that you can add 500 MB for 5 pounds with a bolt-on from O2. That's cheaper than paying some high fee per MB 'extra' you use, but still not cheap .
For comparison purposes, I am using a dongle with Three on this laptop, at 1000 MB for 50p, so I get 15 GB for only 7.50 a month. OK, it isn't an iPhone, and the data transfer speed might be slower, but it compares quite adequately with landline ISP prices, and makes the charges for data for iPhones look too steep for me to be tempted {even if I had the cash for an iPhone}.
If I wanted to have some music from say Jazz FM then I would use up the 500 MB in a few days listening say 5 hours a day. That's perhaps an unusual 'use' but imagine some people listening to the football or cricket via an iPhone if their company blocked audio through the company network (not a bad idea for the company)... FiveLive can use 25 MB an hour (and that's a speech channel in mono, not stereo music).