It depends which Devon station you would like to go to. As an example, I've looked up the journey from Norwich to Exeter Central, which is a direct train route. This would take about 5 1/2 hours and a single ticket, off peak times is around £121. This fare is tied to a specific travel time - an 'anytime' ticket would cost about £160. If you want a return ticket, the price is actually very similar to a single, though again if you want a cheap ticket you have to travel at particular times.

The best way is for you to have a look at the National Rail Enquiries website and type in the exact station you want on the 'plan your journey' section at the top of the page, plus the general time of day. If you want a return ticket, just click the 'add a return' button. There is also a Help section and a number to call if you want to ask any specific questions.