The last time I checked an extra could earn anything from a hotdog and a slap on the butt (on the low end), up to $100/day and very polite treatment (on the high end). I don’t the going rate, and it varies from town to town, but don’t be surprised if it’s not all that much. Remember: you’re doing it for the experience.
Under no circumstances pay a company to teach you “how to be an extra”. I consider this unneccessary, and many would say unethical. I have firsthand experience with a company called “Reynolds” based in Atlanta who were offering (for a fee) to teach people “how to be an extra”. And if they found you work as an extra they would extract a portion of your daily earnings as an “agent’s fee”. I hope that they have discontinued this practice, but take them to task for having deployed the practice in the first place.