I work in IT Security, we charge £1200 per day (9am-5:30pm) for me to essentially sit there and press NEXT when installing software. However, as someone said above, you are also paying for my knowledge / experience plus also the knowledge / experience of the company I work for plus after install promises. That £1200 goes towards my salary, my expenses, profit margins, sales costs etc so I appreciate it's probably a poor comparison.
I guess at the end of the day, it's down to experience. A joiner would be worth £300 to me if he was doing what I wanted and to a level I would expect from an experienced tradesman, depending on the type of job of course!