Look up Adventure Seaplanes in MN. I got my commercial add-on ASES for $650. The guys there are awesome, I HIGHLY recommend them.
My checkride consisted of T/O, steep turns, taxi, step taxi, step turn, plowing, normal landing, and glassy-water landing. Sailing, too. Oral was quick... Maybe 20 minutes worth of questions you've heard before.
I got my commercial AMES in FL for $2,500, but that cost has gone up for sure... I was the last guy who got that price. Checkride was the same as ASES. Oral consisted of three questions: How many plugs does a Widgeon have? Why would you step taxi? And one other, which I forgot. After that it was "Let's go flying!"