Facebook originally operated under "thefacebook.com" and bought "facebook.com" for $200,000 in August 2005.[1]
Looking at Crunchbase, they had raised around $13M at that point, so I'm sure spending $200,000 seemed like a lot at the time.[2]
[1] http://mashable.com/2006/08/25/facebook-profile/
[2] https://www.crunchbase.com/organization/facebook/funding-rou...