3G? Don't you mean 4G?
3G still works but its slow compared to 4G.
4G costs a little more though.
But it depends on where you're located and what 3G/4G suppliers there are in your area/locality.
You could get a PAYG 3G SIM pretty cheap with Tesco mobile if you're in the UK.
That's about the cheapest 3G data plan.
But GiffGaff do some pretty cheap 3F PAYG SIM only deals too.