If you watch a Slingbox from a computer within the same local area network, then the streaming data only pass through your router and do not go out through the Internet, and so Comcast would not log that data within its 250 GB monthly limit. I assume that all of the computers in your house are talking to the same router, as is the Slingbox. So they would all be in the same local area network.
I find that my Slingbox, when streaming standard definition (not HD) video sends a bit less than 1 GB per hour. So, if watching on a computer outside of my local area network, I could watch around 250 hours of TV per month via the SB. All of my other uses (including Youtube) combined involve only a few GBs per month, at most.