We just returned from Panama after staying for a week. We did not watn to rent a car and took taxis everywhere.
First off, from the airport to our hote (RIU) it cost us $33 for two adults/children with 3 bags and $30 on the way back.
Locally, the fare is negotiable to do so ( a good yardstick was to offer half of what they asked and then agree somewhere in the middle). This will give you a better fare, probably still way higher than what the locals pay, but much cheaper than what you would pay in the States.
We found a great cab driver, Geraldo (cell,6684-0782) who took us to Gamboa rainforest ($70 return) and to Buenaentura (Farallon) for $140 return.