I recently started my own fitness based company, tailored to athletes, and I've been an athlete in organized sport for 14 years.
It really depends on what you're looking to work on. For example, if you want to gain mass, you're going to need access to weights. If you're strictly looking to do cardio, you could easily bypass the gym and go for a run 3-4 times a week. There are so many gyms out there now that you could easily get a gym membership for as low as $10 a month, but on average I'd say you'll be paying around $30 a month. Then again, there are gyms that offer a lot of amenities, that can charge as much as $200 a month. Again, this all depends on your needs, budget, and comfort level of working out in front of strangers.
I'd love to hop on a call if you'd like, and talk more about this, or answer any other questions you have about the topic. Good luck! :)