It's just very difficult. You have to be prolific and good, and you have to do a lot of networking. I only personally know of two people who made a living freelancing for the RPG industry, and I've met a lot of RPG writers. No one should jump into the industry expecting to pay all their bills through RPG-writing alone. It's hard to say.
Every company is different.
They'll pay you differing amounts depending on your experience and how much work you've done for them. Now, many RPG books are written by multiple authors. Apart from main rulebooks, most of them are not huge tomes of information. So say you get a contract for 50,000 words - a rather large contract. That may seem like a lot, but keep going.