You can get started by repurposing a computer you have and just adding cards. As long the motherboard has PCI-E slots and at least 2 GB ram it will work. Add risers and maybe a bigger PSU and you are golden. You can also find many used Dell/HP towers for <$100 on Craigslist or at Goodwill. Here is a good thread to read for other ideas.
https://bitcointalk.org/index.php?topic=1955358.0Thanks, this was very helpful.