You can just say it was a competitive market rate. If they pry further after you made it obvious you didn't want to answer with a specific, they're now the comb

You can just say it was a competitive market rate. If they pry further after you made it obvious you didn't want to answer with a specific, they're now the combative one, and it's easier to say "I'm really more focused on the compensation for this new position, as long as you're offering a competitive market rate then it's not going to be a problem". If they really need a number after that you can just say it isn't something you share to other employers out of respect to your current employer.