This thread was started with that exact same question, so be sure to read through it. Many of us utilize small form factors (SFF) computers that we have bought used off EBay or other merchant sites. BI does not require a lot of CPU power, so you certainly don't need the "latest and greatest" computer and buying used can save you a lot of money. The added bonus is that normally these computers were sold with Windows Pro OEM software preinstalled on them which means you can usually install Windows on them without actually having to purchase a Windows license (even if the computer doesn't come with an OS installed). Even if you have to add your own system HD, the license is associated with the motherboard of the computer, so just connecting the computer to MS license validation system should validate the license if it had Windows OEM installed on it from the factory.
Long story short, you can easily find a suitable used computer for around $200-250 shipped. Add a suitable secondary HD for BI recordings and the BI software and you should be "good to go."