Re: Two things
The true cost, like for like, is unlikely to be an additional 100% of salary. I have to do these calculations very often - including all sundry costs. So you are looking at much less than that but it varies per employee.
However even if it was that high then that is where you get similarity in costs where the contractor can cover their own sundries, such as holiday and sickness and has much more flexibility on where to spend it plus the additional tax benefits and the extras you can put through the company accounts.
I've done both and there are pros and cons to both but the income easily covered my holiday time and any sickness, it was just I had less flexibility as I would have to take holiday between roles unless on a long term contract when I would build it in to the contract or supply a replacement for that period.
The biggest issue with contracting was travelling to jobs as I didn't want to travel too far in the latter stages (when I got older with family) so it limited my choices of roles. When I was younger it didn't matter because even permanent roles I would live where the job was.