Your best bet is to rent the equipment and/or the idsn lines/ meeting space. The real cost is the isdn lines. (Yes you can use IP, but don't count on using your T1 for email, web, irc, etc while video conferencing. And from what I've seen most people use isdn.) IIRC ATT charges 54 cents a minute per line. Say you have a tipical conference, good quality video and audio- 3 pairs of isdn lines. For one hour, your isdn costs are almost $200. That doesn't even include the bridging service charges. Which you will need especially if the conference is international.
Figure at least $200 a/day to rent a polycom unit. That is supplying your own isdn lines. Most A/V rental houses have a 3 or 4 day week. ($600 - $800 for 7 days.) At my old job we charged $250 an hour with a 2 hour minimum for using our in house isdn lines, polycom unit, TV, conference room.
Video conferencing can be a HUGE headache. I've done way too many of them. :-) Don't wait for the last minute to organize the conference. Waaaay too many people with out isdn lines call 3 days before looking for a unit and they haven't even called for isdn istallation. It's not going to happen.