It usually is, for investment bankers. But that doesn't guarantee they'll work in your interest. Investment bankers selling a small co to Microsoft might sell it super cheap, in the hope of earning good will with Microsoft that could be cashed in later in much bigger deals.
Also if it's anything like real estate, their incentive is more to close the deal than to get the highest price, freeing them to move on to the next. A difference of a few percent to you the seller is huge, to the banker its a small percentage of that few percent. He'd rather just get it over with and move on.
They can go as low as the seller is willing to accept, who ultimately has to sign off on the deal. However, the broker can essentially make stuff up to make the seller think they should take the deal.
