You might consider doing what I did.
Get a T-Mobile Mobile Broadband dongle and stick the SIM in your phone.
Suits me just fine - I rarely do more than 10 minutes of calls and a couple of dozen texts a per month.
But I need/use a LOT of data
So I took their Mobile-Broadband Max package @£15pm - calls cost 20p per min & texts are 10p each.
My monthly spend is about £18 for using the phone exactly how I want (little chat - lots of eMails/streaming/browsing/file transfer)
Also if I know I'm going to be doing extra data T-Mob will happily bump my account to a Max+ (=10GB data) for one month for £5 extra.

I'm using an unlocked e71 - and it tethers quite happily to my laptop & N810.
Should be a doddle to move to the N900