Mac OS X is not a Time-Sharing system. Mac OS X does not easily support multiple simultaneous Users with a full Graphical User Interface.
You can have multple accounts on a Mac so that several Users can have complete control of everything after they log in. That will allow them to share the computer, but not at the same time.
Using a central Mac OS X Server, you can store ALL User files on the Server and set up a computer Cluster where any User can log in at any computer and have Instant access to their files. This is used in schools to provide a pool of computers -- where any computer from the pool is instantly available to any student.
Multiple-display support on Macs is used to build an larger Extended Desktop for the currently logged-in User, like this:
If high cost is your main concern, look at the Mac Mini. It is like a laptop computer than is not portable, but starts at US$599. Bring your own display or use a Hi-Def TV; bring your own keyboard.
Also, less-expensive Refurbished Macs from Apple are in pristine like-new condition and are available for sale under the same warranty as new.