How much to charge for a pre-built computer

[James Colannino]

Hey everyone.  I don't have much expertise in the field of building 
computers as a business, and recently someone told me they would be 
interested to have me put together an old computer that they could use 
as a low-end server.  I bought a used computer that was supposed to be 
just for "parts" for $20, and found out by pricing the individual 
components inside that it was worth at least $50, but most likely about 
$65-70.  The computer works fine as it is, so there's my old system 
right there.  He wanted me to pre-install Linux and a few third party 
applications he could use for his webserver such as a firewall and all 
that.  The thing is, I don't know how much to charge.  I'm an honest 
person, and don't want to overcharge, and I don't know how much I should 
charge for the system added to my time installing all the software and 
configuring the options and all that.  Should I charge a little less 
than the full worth since I got it so cheap, and how much should I 
charge for my time and efforts?

James