So what does it cost

Aside from the time taken to fix the problem there are 3 factors to consider;

So why the call out fee?

Well, i could just 'hike the price' to cover my costs & say 'no call out fee' but i try to offer the best service i can for the best price i can.
Ignoring the cost of running a van (which IS substantial) when i go out on jobs i incur travel time for which i am not paid (this usually ends up being 30 to 60 minutes round trip). To offset this loss i charge a call out fee, which INCLUDES the first 1/2 hour (unlike my plumber who charges for the call out AND all the time).
However I also offer the facility to have my customers drop their PCs off at a time that suits us both, which saves them money (& me time).
Some people just prefer the convenience & have me visit them every time they have an issue, others especially laptop users who live locally prefer the drop off service. If a job is involved or you cannot be without your work computer during the day we can make arrangements for me to do the job 'after hours'