OK, fine. Let's have a look at the maths involved. The following is based on the shortest route available, as dictated by Railmiles: 920mi 50ch, and also on (slightly out of date) access and fuel charges available from Network Rail.
A 142 has access charges of 3.95p per carriage per mile (a 143 is actually 0.02p cheaper), which when both carriages' costs multiplied by 920.625 gives a total of £72.73 (to the nearest penny).
This thread, from a couple of months ago, details fuel costs, and suggests that one litre costs approximately 70p for TOCs. There are 4.544 litres in one gallon, giving a cost of approximately £3.18 per gallon. If a 142 can manage 3 miles per gallon (DMUs tend to do anywhere from 1 to 3 and I would expect a 142 to be at the top end of that, although I'm not certain), we would require approximately 307 gallons of diesel, which would result in a cost of approximately £975; add to this the access costs and we are looking at about £1,050.
Don't quote me on this figure unles someone can verify it, but without looking I would have estimated about £1,000 anyway.
So if 30 people were to take part, allowing for administration costs, the hope of having a more interesting route that that suggested, and more, I would advise a cost per person of roughly £60 (which would be £1,800), on the assumption that the TOC(s) involved would be prepared to lend us a unit without charge, give the charity implications (and publicity). If they wouldn't though, well...
(I REALLY like the idea!) - I'd probly end up just designing n printing all the publicity though (I sadly have no specialist train-related skills 
)
