EDIT: This is the cheapest complete mini system I've found, which still excludes the NVIDIA video card:
http://www.mini-box.com/Mini-Box-M300You'll also need memory and storage of course.
There are better solutions available. Intel makes/sells Atom based Mini-ITX boards for under CDN$100.
A suitable NVidia card should cost no more than $50 or so, though my search-fu fails me at the moment on finding the PCI versions that are out there.
RAM is about $12, and storage (flash card + usb dongle) is about $20 for a diskless client.
Then just find a case+psu to your liking .. lots of those out there, cheap and expensive alike.
Cheers