What does this cost?

Typically, the seller, from the proceeds of your purchase pays for all the service that you, the buyer, receive from a professional buyer's broker.

Then whom does the buyer's broker represent?

By Florida law, an exclusive buyer's broker is required to represent and protect only the interest of the buyer in a real estate transaction. He or she can in no way whatsoever represent the seller.

Is there any reason why I shouldn't retain the service of a professional buyer's broker?

No, there is none. Usually, you are already paying for these service when you purchase the property.... so why not take advantage of them?