There's a lot of good reasons for building a PC. But doing so primarily to come up with the lowest cost 'box o'chips' isn't one of them.
You may be able to offer your client
better value at specific (usually $700 USD and up) price points. But once you go below that you really can't compete with volume OEMs. Especially when you consider a copy of Win7 costs them less than $40 per shipped machine.
For that, the solution is to get a better quality low-end machine from one of the 'big boys' and judiciously upgrade and tune it for your client. I've had good luck with HP and Dell for that. Catch them on sale and you'll never be able to match their price for what you get.
Many of these inexpensive "business" machines work quite well once you clean out the crapware and do some performance tweaking. HP in particular benefits hugely from that. I have a client that just bought a few inexpensive HP biz desktops. Performance was
meh despite having fairly powerful AMD multicores. Some driver and OS updates combined with wholesale crapware elimination turned these little guys into a pack of very sprightly workstations. Add in an inexpensive RAM upgrade plus some system tweaks, and now they almost sing!
Which further reinforces my conviction that on low to low/mid level hardware, setup and system maintenance are as (or even more) important than the hardware. This is where a small computer business can best add value.
You can't compete with the big OEMs on hardware or OS pricing. They can't compete with us for doing all the things you need to do to pull maximum performance out of the hardware. It's too time consuming for them. That's why they take the easy way out by cutting hardware prices to the bone; do the simplest (bordering on braindead) system setup imaginable; and boost revenues by collecting bounty payments for installing junk.
For them, it's all about volume. But for us it's all about performance and customer service.
To which I say:
Vive la différence! 