I've been on every network in the UK, and most of the french ones. . . In Europe you get a basic Data plan included, I get 1mb free/Month with my pay as you go simcard (Subject to topping up) and I pa about £2 to £3 for every additional Megabyte.
Several companies have "Data-Only" Tariffs where they give you between 250, and Unlimeted megabytes a month. . Vodafone's Data tarrffs: £25 a month, 250 free, £2.50 every MB after: £60, unlimited access.
But then you can't use them as a phone.
Only really good for a CF GPRS or 3G card, or if you have a spare phone you don't mind sacrificing in the name of Zaurus.