It is reasonable to ask for up to a total of 10-20 GB with little or no justification. Beyond that we generally would like to see some sort of explanation of what you need the space for. When there are larger quantities of data involved, it can become important to locate the data on the best fileserver for the task, understand the network requirements, and determine the backup strategy.
While it is true that the raw cost per GB for typical PC hard disks has become quite low, the disks we use on our fileservers are more expensive since they must meet higher standards for reliability. In addition there are a number of "hidden" costs associated with shared storage, such as management, backup, networking, the cost of the servers, etc. We put all critical data, including users' home directories, into RAID disks systems, which provide even greater reliability but at significant additional costs in disk overhead, smart controllers, redundant powers supplies, and so forth. Altogether these can raise the cost per GB considerably over that for a low-end disk at Fry's. When one multiplies by the number of UNIX accounts on our system I'm sure you can understand why we don't automatically give every new account large quotas.