Agents aren't like Tesco. There isn't a price and that's what pops up at the checkout.
Your project manager may have a figure but that's come from a half-baked calculation by Finance who may have all sorts of motivations pushing them in different directions.
There are rate card discounts; bulk discounts, free services, entertainment etc. Your agency contact may not know because generally there are different people for contract and perm so they may not individually know the overall turnover/discount etc.
Ask around by all means. Lots of people are happy to talk about such matters. But be careful - margins are usually a lot lower than they first appear.