So, I can answer this pretty well since I had a writing mentor in the type of showbiz I work in. The truth is that a lot of mentorships don't cost a dime, and to a point, that's how it SHOULD be. But, let's say that you go the hiring route first. 

If you hire a professional business mentor, then you're going to look at $500 a month at a minimum. Some mentors, such as that Mark Cuban guy, will charge way more simply because they are celebrities who have a lot more fame and success. It's not unusual to hear of some Fortune 500 mentors charging as much as $10,000 a month. 

For that money, you better be getting some quality advice and real in-depth looks at how you run your business. If you don't, then you are going to be SOL and you're going to be out of money. It happens a lot more than you think. 

But let's say that you get one for free. It's possible, especially in fields like showbiz or music. There are perks and pitfalls here. First off, your mentor will likely be a major friend and ally of yours. It also won't cost anything for advice. However, you might end up getting what you pay for. It's kind of a crapshoot, but it's often worth it. 

Want to know how to get a mentor for free? Here's the scoop:

1. Network, network, network. Reach out to the movers and shakers in your industry and ask them for advice. In most cases, they'll be happy to help. 
2. Don't push people who don't seem interested in helping you. This is a good way to make people want to steer clear of you. 
3. On a similar note, know when you are asking for too much. Unless you intend to pay them for their time, don't ask them to go through your books or papers. 
4. Recognize and respect their time.