Yikes- $800 for a T1?!? That sounds like pre year 2000 prices. Even in extremely rural areas I don't think I've seen T1s cost that much any time in recent memory. And having 100 workstations on a single T1, wow...that's another pre 2000 flashback.. Not sure how this has been sustainable on any level in this era of graphically intensive websites and streaming media, etc.
If you decide to keep the T1, I would recommend immediately finding a new carrier and obtaining an updated quote. Also, look into options for EoC (Ethernet-over-Copper) which is a more efficient, scalable, and less costly T1 type of technology. If you decide to use Time Warner Cable for the main Internet line- perhaps consider implementing T1, bonded T1s, or EoC as your failsafe/backup connection.
*Please contact me directly if you need any price quotes: