Okay, maybe this is a soapbox item, but I've been to too many companies with only one domain controller.
Most of these companies have a full system backup running, but upon complete failure a restore will take a few hours minimum. One such outage and the cost of labor to pay an IT professional to restore a domain controller is about break-even, with an example price of $700+ for a Windows Server license and the cost of a cheap server or desktop. That of course doesn't really cover the true cost; if you have a company of twenty-five employees sitting around waiting for the server to come back online, now you're really talking money. With another domain controller present (with sufficient resources), the clients may never even notice the difference should a primary system fail.
Unfortunately, many small companies look at the short term cost of items rather than focusing on the immense cost of a prolonged outage or lost productivity with subpar equipment or dated technology. But in tough economic times many of these choices are made for them.