The company I work for, is not very big, but has two dedicated T1 connections. On average, we get around 3MB downloads, and 1MB uploads. The company I work for hosts several different Websites (14 as of right now). I think they pay around $1500.00 a month- and I believe this also covers VoIP phones that we have.
For a standard busness connection, you can get away with 1-$200 a month for around 3-8MB downlaods, and 128KB-1MB uploads. This is for your basic home-based business. I've been thinking of building my own server and hosting my own Web site as well, due to my host having problems. If I can remember, I'll PM when I do this, and let you know what it runs me, and what services I get.