The Sun promotional literature[1] refers to 70%+ cost savings on their units over comparable systems from EMC (the old school market leader). It was the fastest growing product at Sun, as it allowed companies to cut down storage costs using a model similar to what Google pioneered with Big Table and GFS.
[1] http://www.oracle.com/us/products/servers-storage/storage/un...
From Google's perspective making use of these massive storage capabilities is a plus for them. They are able to do it at much lower cost than just about anyone else, so when they find a business case where they can leverage their storage capabilities they are smart to do so because it can translate into a competitive advantage. When you've got the fastest dragster on the strip you don't go around figuring out how to use it the least, rather you try your damnedest to race every car that you can.