Wow, $175? Are you salvaging parts and OS from another computer?
Not really. I re-used my existing 22" monitor, and a keyboard and mouse. Everything else was acquired. Here's the basic breakdown:
$30-ish for HP mini-desktop with Intel Core Duo (2.1 to 2.4 GHz) and 1 to 4 GB RAM (has PCIx-16 slot for video card -- that's critical) (eBay)
$20-ish to upgrade from 1 to 4 GB RAM (or if it came with more Ram, the above will be higher and this lower) (eBay)
$30 for least expensive new SATAII HDD (NewEgg)
$70 for GeForce 640 GT card PCIx-16 (I got a $40 rebate one 1 of the 3) (NewEgg)
$30 for Windows 7 home premium COA (eBay), downloaded installation ISO from MS web site
Of the 3 mini-desktops I bought, 2 had CDRW/DVD-ROM combo drives and one had DVD-ROM.
I also spent $11 to get a multi-card-reader that would fit into the floppy bay (for only the one I'm keeping).
You have to know enough to get the proper RAM type for the machine, and to buy a machine with a PCIx16 slot and then a video card that goes into that type of slot. FWIW, PICx16 1.0 can almost always take 2.0 cards (which are all you can find new). Almost everything else is so standardized these days that you will be able to slap it all together and it will work.
Note that the eBay stuff was used (of course) whereas the NewEgg parts were new (HDD and video card).
I forgot $1 each for a special type of screw needed to mount the HDD into the "modular bay" of the machines. No biggy there.