Steam excursions are generally quite expensive. (Operating costs are very high; in particular, Network Rail charge the operator a massive fee for providing a 'path' for the train).
This site tries to list every scheduled steam excursion in the UK. Click the links, in the right-hand column, for details of the company running the tour (which usually includes a further link to the relevant website):
http://www.uksteam.info/tours/trs07.htm
Unfortunately, there don't seem to be many tours close to Buckinghamshire.
For something cheaper, you could always go out for Sunday lunch in the Chinnor area, and then take a ride on the Chinnor & Princes Risborough Railway:
http://www.cprra.co.uk/
You could also do something similar using the Leighton Buzzard Railway but, to me, narrow-gauge railways never seem to be 'proper' steam trains:
http://www.buzzrail.co.uk/
Chris