Browsing websites hardly takes any bandwidth. Videos however, do.

I dont use YouTube on the phone, and am connected to Wifi at home and work. On the road i'll use 3G and since September I have used 300MB down and 40MB upload. That is browsing websites, checking email.

For videos you can kinda calculate how much you need. Each youtube video is about 25MB. Multiply that by the number of videos you want to watch and there you go :-)

ps. In most countries its possible to get an iPhone plan, with unlimited data and free iPhone. I have such plan and only pay 30 a month. Maybe you can find something like that? On bandwidth alone you will save, and the included minutes/texts will just be more savings.