Depends on what network. I just have order a sim only from o2 online paying £30 for 800mins and 1000 texts plus £7.50 for unlimited data.
Heres the link if you want it http://shop.o2.co.uk/tariffs/simonly
The cheapest would be 22.6 (inclu bolt on) but are there any other cheaper then that as i don't quite use the call and text (receive messages 1~2hr later)