I just snared a Magma 4-slot PCI expansion chassis for less than $200. It came with a PCI host adaptor.
Since my plan is to run Pro Tools on a used Macbook Pro with an ExpressCard slot (for expandability), all I need is to purchase an ExpressCard host adaptor. Then whatever I put in the expansion chassis will work with the Macbook.
With more and more PCI Pro Tools cards coming onto the second-hand market, this could be a semi-cheap way to get Pro Tools HD.
For now, the unit will serve as an external RAID drive. It holds four hard drives and I also just got a 4-port SATA card.