In my experience, Personal Trainers can cost between $30 to hundreds of dollars per workout. The price depends on the gym, experience level and the tools like Recovery Monitoring your trainer may use.
Choose a Personal Trainer who meets your specific goals
I chose a Personal trainer that specializes in football centric training, for a running back. He monitored my diet and Recovery day to day. My workouts were designed for strength, with a focus on lower body. He took the time to show me proper form in the movements and with video tutorials. We also did workouts that simulated football games.
My 60 minute workouts resembled all out effort for 30-45 seconds with 30-45 seconds of rest. The concept of getting my body ready for the same type of effort it would endure was a new concept to me and really showed the value of a trainer experienced in what I needed. It prepared for me for the position better than I dreamed, all because of the small gaines from experience.
Proper Form
Proper Form is one of the most important aspects that a trainer can help you with, without tutorials on this, you are not getting the best gains possible. Form is very important in reaching your individual results. It is vital you choose a personal trainer that has experience with form, specifically for your goals.
You should select a personal trainer that specializes in your particular goals, especially for their experience in proper form. You wouldn't hire a plumber to help you with your electrical wiring.
Bonus: Find a personal trainer that uses video tutorials for Proper Form. (example for nitric-oxide dump)
Recovery Training
Recovery Training is as important as proper form. Without it you are losing a lot of performance gains. When your body is not recovered fully you can even do more damage than good. You should look for a trainer that has a proper recovery protocol built into the workouts they use.
Bonus: Find a personal trainer that uses real recovery training to get the best from your daily and weekly workout plan.