A hard drive will be "relatively" expensive compared to computer hard drives off the shelf...

but...

Once you install you will be good for another long period of time (first one lasted 6.5 years) and if the system does what you need i would just replace the drive.

If you are looking for additional features, functionality etc now would be the time to consider an upgrade. You could put in a CS 50 and reuse all of your phones...... and you should be able to reuse any epxansion cards if you have them.