We need to know the following to advise properly, as the answer depends on:
* Do you have a family and friends railcard?
* Do you have/want an oyster card?
* What time you need to arrive?
* Are you prepared to use advance tickets (consider that these would be rendered invalid if your return flight was late, presuming you are flying)?
If you want the advance tickets (ie booked onto a train) then you'll have to wait till approx 12 weeks before travel to be able to book these: these could work out very cheap (eg with a friends and family railcard, and oyster cards each it could be as low as £12.45 one way), but as noted above this might not be the best option if flying.
There are also huge price variations between peak/off-peak costs, so we really need more info.
As a generalisation, however, you'll probably find the best fares travelling with Chiltern Railway via Banbury to London Marleybone, then taking the tube.