In the UK most 'pay as you go' tarrifs allow you to add 'bolt-ons' so for £5 a month you can have 'unlimited' data (up to 500mb usually).
Pretty much every contact tarrif also allows this.

So depending on your location you might be able to get quite a good deal on data.

I'm on Orange (UK) and they made a cock up on my bill and I now get data completely free.