Bus service in Bocas is somewhat irregular, but there is a daily shuttle that will take you to Drago Beach and back. Times change, so ask at your hostel when the shuttle departs from the main park.
If you're heading across to Aqua Lounge or any of the nearby island beaches, water taxis ply the route regularly. Cost should be between $2 - $4 per person, depending on the time, the size of the group and your level of intoxication.
There are also quite a few taxis plying the streets of Bocas. A trip within town won't be more than a dollar or two, but head to the further destinations (such as far off beaches) and the costs will mount rapidly. Bargain hard.