To bill or not to bill?

Sort of a long range question here.

I am delighted to have been invited back to my old seat of learning in Oxford (OK, the University formerly known as Oxford Poly). Obviously with long student holidays nothing will happen now until well into September, so it shouldn't impact my £100 budget (I'm rather hoping to have cracked it by then!) and I am quite happy to deliver the presentation for free, particularly as it is basically an advertisement for the course. The big question, should I go for traveling expenses, or should I put this down to marketing with an element of social pay-back?

There's plenty of time to decide..