It depends but I usually charge no less than 50$/ hour. That's if you want someone who fully understands GraphQL, Redux, Sagas, Thunks, Generators, Offline Persistence, etc
The biggest mistake you can make as a contractor is underpricing yourself. Not only does it hurt the market but you can find yourself in situations where you can't work enough hours to cover taxes, downtime, benefits like health insurance, much less a vacation. Then you start making mistakes like overbooking yourself to make up the difference and do a crappy job with all of your projects.