A success fee is a compensation structure paid to an investment bank for successfully closing a transaction. The success fee is usually calculated as a percentage of the company's enterprise value, and is contingent on the completion of the deal. While the success fee may appear to be high, the fact that it is contingent on closing the deal aligns the investment banker with the seller's interest.
The investment bankers take on some risk given that they don't get paid if the deal doesn't close. The fee itself should be a fraction of the increased purchase price that a top-notch investment banker should be able to get from prospective buyers.