If you are informed enough, and you can guess with decent accuracy how long it should take you to do the project, nail down a contract and bid the job. If thier parameters are likely to change, say add more features as you go or change the spec, bill by the hour. To bill by the job, you need them to sign off on what you are expected to deliver down to the last detail. This however has a benefit: If you run under the time, you make out. If you go over, they make out in that it doesn't cost them more. Everyone is happy.
Starting out with 2-5yrs experience, bill $50 an hour +/-$10. Remember you have to pay taxes on that, so put 40% away in a savings account for later. As projects get larger, or are based on older projects you have done for them, you can tweak the price up a bit more. Less than two years, stay under $40 and don't quit the day job, moonlight.
Good luck, and contracting can be a rewarding, but yet sometimes painful job when things go dry. Be sure to ask a few people in your area that you can trust what they get paid or pay out for contract work. (bid a tad under at first until you improve your name and reputation).