Peanuts, I'm afraid, if it's from the seventies.
If it were older with some historical value, that's different - anything military, particularly with regimental markings, would carry a premium. Whippy also made a lot of sidesaddles and in good condition they make serious money.
It might be worth more to Whippy collector if it's an unusual or rare model, but few catalogues survive (I've never seen one and I keep an eye out for that sort of stuff) so it'd be hard to find that out. Otherwise it's just a 40-year-old, used saddle with a famous name.
Sorry I can't be more optimistic :(