Cost of server for commercial purposes catering to at least 100,000 users?

What would a server(s) which caters to at least 100,000 users and which provides at least 1 million GB(is that one terabyte?) of space cost to install and maintain.

Depends on the LEVEL of "use" each of the 100,000 users would out on it
WHO you buy it from and what level of support you expect after purchase

I don't think thats a question for Yahoo, its a question for a company such as the one I work for, someone who can validate your pre-sales needs and expectations.

If your looking at that many people you need to look at many things not just the server, you need to incorporate total redundancy so that the users will not be inconvenienced if any hardware fails.
In situations such as yours I would use 300gb drives (the new 2.5" SAS - Serial Attached Scsi) and use Raid 5 at the very least for the data but use Mirrored drives for the Operating system. The new HP servers can take 9 of these drives without any expansion so so should be fine for storage.
For that many users your looking at Gigabit network at the least if not fibre channel to your Wan.
From there you need to incorporate redundant power supplies and if its totally vital it is always up etc then you should incorporate a totally mirrored server so you essentially have 2 servers identical - linked by Fibre channel and working together.
That would give you the highest protection against failure but as with all these things.... at a cost !

There are many solutions you can look at, Blade servers for example use servers on a single circuit board, you could purchase 2 or more blade servers, link them all to a storage array so you only have 1 cabinet of drives, this would balance the cost better.
A lot of the cost as well would be in the comms equipment to cater for such a large number of users - decent equipment i.e Cisco would cost far more than the servers themselves but would be vital if you want performance and reliability.