I've been having this conversation with coworkers a lot lately. In San Diego, a senior sw guy can expect to pull $120-160k(there are outliers... I have a friend making $240k, but it's unsustainable) as an employee with benefits. As an independent contractor, you can make around $75-120 / hour. As a SW consulting company you can get $120-180 / hour. These are all rough numbers based on talking with people.
I don't know why there is a disparity between what ICs and consulting companies make but it seems to exist.
The amount you charge also depends on many factors (difficulty, on-site vs remote, total hours per week, ability of company to pay, etc).