varies from gym to gym
depends.
like 24 Hour Fitness, sells 5 sessions for rougly $289 or so, 10 would be $590 and yadda yadda.
Yes, I'd recommend that if your starting out in the gym, hire a personal trainer. Try for like 10 sessions, & if you like it. pay for more. =] I would say, if you didn't have someone to guide your way in the beginning, you will probably be wasting your time in the gym.
& also
with personal training:
your body measurements as well as body fat % will be recorded and every few weeks, it will be retaken to see what your progress is.
&
you will receive a simple food/diet program that custom fits your lifestyle, if you follow what your trainer tells you in the time alotted. you can reach your goal faster than if you were to do it ALONE.