Basic charges and the upsell.
I tell my customers that I will gladly look at their slow machine and then tell them what I recommend. . . for $50 . . and I do just that. I take a look and then call them with a recommendation. The call goes like this "I found 2GB of junk on your computer, you are infected with one really bad spyware program, your registry has 62 entries that are slowing it down, you have 5 programs that slow things when you start the computer, your WinXP needs to be updated, your virus program is expired and needs replacing . . . " Then their "customized" clean up job costs $50 or so but since they "know a friend of mine" or something, they get the $10 discount (which I put right on the bill so they see what they could have paid). They always tell me to do the job. That is $90 in my pocket. When they get the machine back they ALWAYS get a written estimate for an upgrade, usually memory, sometimes video card or software. 40% come back to me for the upgrade work.
After looking at the BestBuy and FireDog and Staples menus of computer shop work I came up with my pricing scheme. It come in at 50-70% of what they charge, so I can take the money in good conscience.
This pricing might seem high but I am just outside of NYC.