Re: How much money should i charge?
If you are currently employed, freelance rates tend to be about 2x of your hourly rate at a full-time employment, because freelance works do not require benefits such as vacation or insurance. If the project is particularly interesting to you, you may find yourself happy with charging less money. Do be warned of start-up companies or personal projects that promise "publicity" as part of the benefit though. Many of these companies or projects do not end up getting any publicity at all. Do some background research and think about whether you'd put actual money behind the project when in doubt, because by being under-paid, that's practically what you are doing.
Once you determine your hourly rate, you need to figure out how long it takes you to do a job. This may seem daunting but all it takes is actually timing yourself when you are doing any kind of work. Exclude the time when you get sidetracked and start browsing online, etc, but include research time. Then it's simple mathematics to calculate a per-project or per-piece rate. Don't forget that if you are getting paid over paypal or similar online services, there is often a fee. It's good practice to mention who pays that fee up front to avoid bad surprises later.
Lastly - payment and delivery schedules. You need to agree on when the payments are made upfront, as well as figure out when the assets are expected. It's wise to limit the amount and scope of revisions as well, and state that any revision beyond agreed ones will be charged at your hourly rate. If a job is needed very urgently and require your full and immediate attention, it may be classified as a rush job, and you can charge more. I don't really know what the standard on rush jobs are, though.
If you've never worked with your employer before, you may want to do a 50% up-front and 50% before delivery of final, usable asset kind of payment schedule to protect yourself. If they are hesitant to pay the 50% before any work is done, you could schedule for a delivery of sketches or untextured models, or a similar small chunk of work that shows your commitment to actually do the job without risking doing too much stuff for free.
Hope this helps!