I'm not sure there are any low cost options for this. Something you'll need to consider is whether each client is "SAN-worthy" of a local backup based on the amount of data and the backup window involved.
The options avaialble other than Shared Storage Option is either LAN based backup, or server-free backup.
The idea behind LAN based backup is to consider using your SSO media servers to also do IP based backups of other clients (in addition to the local SSO backup) in different LAN's than the Prmary backup server. This spreads out the IP traffic across multiple ports/switches, rather than having the Primary do everything, but this depends on your network topography. The cost of this option is remote agents which you may already own. You could get the CASO option (more $) and centrally manage all the media server jobs from the Primary.
server free backup is the concept of either your FC switch or SAN disk controller to create copy -on-write snapshots which can then be presented to the Primary backup server as if they where local drives .
This technology has differnet names depending on the disk vendor
EMC- BCV
HDS- ShadowImage
IBM- Flashcopy
HP BusinessCopy
And usually cost extra $$$$.
Some FC switches use a SCSI protocol called Extended Copy which can write a copy of the data directly to a tape device without having to go through the master server. I know that Veritas netbackup supports this- but I don't believe Backup Exec does. Backup Exec lists a 'Server-Free Option" which used Extended Copy, but I don't beleive it's supported by any of the switch manufactuers yet. (i have seen posts in this forum stating this)
so, consult your disk and switch vendor for solutions. Technology is constantly changing, and maybe they have just the solution for you. If there is something out there that actually saves you money, please post the results!!