Giving a career talk in my old university, how prominently should I tell students my salary?
I agree with you. I feel the request to put your salary on a slide (on the first slide, no less!) is rather unexpected, and quite frankly does not speak highly about the professionalism of your contact. I would decline this, for multiple reasons:


Your salary is nobody's business. Not sure what more there is to say about this.
Students are, for the largest part, intelligent adults. Many would take this exactly for what it's meant to be - rather crude marketing and hype generation. This would detract from the message you actually want to transport, and would undermine your following talk.
Focusing so much on how much you make is arguably not the best way to motivate young people for a specific career path anyway. If you talk about why you love your job (if you do) will encourage more people than a six-digits salary number. More importantly, it will probably encourage the people who will actually be happy doing your job, not the ones who would end up wealthy and miserable.


You can of course provide a salary range as proposed by other answers, but I would not emphasize this point much (and base it on third-party data, such as Glassdoor, not just your own experience).