I think your $10 was perfect (that's what we do). The way I look at it is.... If I don't want to walk and don't want to wait for the boat, the Rickshaws are a convenience I'm happy to pay for. Put yourself in the drivers shoes.... would you want to peddle back and forth with a considerable amount of weight behind you for less than $5? Heck no!!
Most of these guys (and girls) are extremely personable and simply trying to work their way through college. And not only do they ONLY work for tips, they also have to pay to rent the bike (between $25 and $75/night) depending on special events, etc - so they have to make at least that just to breka even!
Jim