Our experience going through the canal as well as in Panama was great. It can be done with or without using an agent, but I recommend an agent for expeditious, trouble free transiting. Saves headaches in finding tires, line handlers, and in scheduling. Probably saved us a few days. Also, can help you not have to pay the buffer and wait for it to be returned.
Hard to say costs, not knowing what size boat you have. A typical 50' will cost about $2100 without an agent with $800-900 or so returned after you transit and don't cause damage. Using an agent pushes the cost up to $1600 or so.
Costs include transit fee, inspection, security charge, buffer (if no agent), tires and lines, line handlers, food for canal authority adviser.
Noonsite is an excellent initial resource to familiarize yourself.
Agents run around $400. Ours was more but we used ours for many things from leading us through Carnival to assisting guests flying in and arranging transportation.