*makes a big assumption*
ok 10gb hard drive
Lets say for arguments sakes all of the sites sold are going to have 1-2gb bandwidth
Lets also say the server does have 512mb ram (over kill for a server bunch of 10mb 1-2gb sites , 256mb would do just fine, even 128mb would probabbly do depends on the CPU explained next)
Now, lets assume you want to go all out on this box CPU/Ram Wise, if you have 512mb sdram then go for a 800mhz P3 or similar.
Now a server like that full of 10mb accounts ONLY would easily be able to fill up the hard drive (remember 10gb is not a full 10gb on a server, part of it is used to hold the operating system ect) so i'd give a liberal 1.5gb to the system files and swap)
I'd venture to say 500, 10meg accounts EASY
However it ALL depends on what each customer does with his 10mb space.
Its possible that you will have a few high usage cgi sites (but NOT likley if all you are selling is 10mb 1-2gb bandwidth sites)
so 500 easy if not more
Now per site varries on your server cost
So give me some server costs and what you would charge for the package and then i'll give you a per site figure