"Fiver" is a British slang term for a five-pound note.
How It's Used
“I’d never acted in my life. I’d even been turned away from the school play for being disruptive. So when the casting agents came to ask me if I wanted a go, I said I’d only do it for a fiver. I knew I wouldn’t get through, so I was just trying to make money.”
—Thomas “Tommo” Turgoose as quoted in The Week (UK), April 28, 2007, p. 10 from an article by Bernadette McNulty in The Daily Telegraph.