How Much to hook up 50 computers and how

G'day Craig,
All depend on the requirements of the network.

Why do you need a router? Are you plan to connect the network to the Internet? or other networks?
What type of servers (file server, database server, etc) do you have? Are the network utilisation of each PC the same? If not, are there any particular group of computers communicate more to each other?

From your post "a sydney educational institution needs 50 computer networked", I am assuming this is for one or two classrooms. If that is the case, I would install a central server for managing the network itself, DHCP/routing/gateway/etc. It then connect to two separate hubs/switches (I prefer switches) for the two separate class rooms. Each class room will have it own subnet, such that traffic of classroom will not interfere with the other. The main server will connect to the two switches and does the routing of packets between the two subnets. The two classrooms would not exchange much data therefore the central server shouldn't be a bottle neck.

Price:
2 x Switches (24 ports) = $AUD 3000
1 x Server = $AUD 4000
Cabling = $AUD 1000 (depends if you cut the cable yourself or buy the ready-made ones)

Sub Total = $AUD 8000

Plus 50 PCs = 50 x $3000
= $AUD $15000

Total = $AUD 23000

Is this an RFQ? I suggest you contact a local reseller/integrator, give him detailed specs and he will provide you a quote that is meaningful. Anything else is pure conjecture.