All available train fares are shown on the National Rail website:
http://www.nationalrail.co.uk/
However, details for next Easter won't be available yet. Train companies have to make their ticket allocations available at least 12 weeks in advance, so it would probably be best to check around the end of January.
That route is operated by East Midlands Trains (to St Pancras), with trains generally running at �-hourly intervals, with journey times between about 1 hour 40 minutes and 2 hours.
An 'Anytime' return ticket, which can be bought at the time of travel, currently costs �70.50. An 'Off-Peak' return ticket, which is also available on the day of travel, costs �49.70. 'Advance' single tickets seem to be priced at �16.00, �19.00 or �23.00, depending upon which service you use. However, the train operating companies often withdraw all advance purchase tickets around holiday periods. Even when such tickets are available, there's often only a very small allocation on each train, so you'll need to book early.
Companies like Trainline.com don't offer tickets any cheaper than the train operators do. They've also got a nasty habit of making errors. (I used to run a train station. Customers often arrived with tickets which weren't valid for travel on any train, due to errors by internet companies). It would be safest to book directly with East Midlands Trains:
http://www.eastmidlandstrains.co.uk/EMTrains/T ickets/
Chris