Figure what you'd be taxed at that salary and add 10-15%. Freelancing also comes with some tricky challenges of maintaining consistent work. There's either too much or not enough at any given time. And if you have multiple jobs going at once, finding time for vacation becomes almost impossible (try lining up a good time to take a break from 3 different projects on 3 different schedules).

In short, more money from freelancing isn't the simple thing it looks like from this article.