If you just call an agent's office, they're only ever going to tell you their standard rates.
Unfortunately, you need to go through the full rigmarole to get the real rates. Get an agent in to do a valuation, talk to them about the service they offer, and once they've committed time to your potential sale, then talk to them about what they're willing to do to win your business - in a friendly and polite way of course!
If they're sniffy and say "I don't do discounts" don't be afraid to just tell them that you're going elsewhere. There are definitely better rates to be found with good agents.