It is recommended that you travel in a group of three or more. It is unlikely that you will negotiate a 30 baht price. OR you may be faced with having to pay 100-300 baht for the boat on your own. At the small boat dock at the river, the boat driver will give you a price. It has been my experience on one trip that the price was changed it in the middle of the river.
Diplomacy is in order here.
If you're a late arrival and the other boats have departed you are at their mercy. My crisis was handled but that's another story. In many cases they will try to drop you off down the shoreline and tell you to take one of their friends motor bikes to the ferry. You can pay the ferry fare on the ship. This is where having dollars works in your favor and you can save money.