I've done plenty of times, $100 is what I charge as well. What you do is say, "Intel, no if's and's or but's". If your confident you know your way around a computer like the back of your hand go for it. Here is a list of things you should do:
1.) Buy all your parts with at least a 2 to 3 year warrentys
2.) Tell them after 12 months you have to charge for any service work
3.) *DO NOT* buy cheap components, always buy top quality parts!!, this will save you ALOT of potential hassle
4.) *DO NOT* buy parts you have never worked with before
5.) *DO NOT* cut corners
6.) *DO NOT* lie to the coustomer
7.) Make sure to tell them that your not a software expert, and that answers for any software related "how to" qeustions can be found in either the softwares manual or website. Also advise them to buy books at any book store on how to work specific programs(Widows for dummies, ect.).
8.) Tell them your warrenty dosn't cover house call's
If you build a computer for someone there is always a potential risk, but the the key factor is trust. When people ask *YOU* to build a computer for them, it is because they trust that when it comes to computers, you know far more then they ever will. If you are confident you can trouble shoot any problem with ease, and feel completely comfortable with computers, then by all means make some money. It's best to come up with your own warrenty statement.
Now everytime something goes wrong with there computer, they call you. It's damn annoying.
If your any good at building computers there wont be any problems you can't solve easily, and a part breaks, whats so hard about the RMA process?