With the release of unified storage (you can read more about it here: http://googleenterprise.blogspot.com/2013/05/bringing-it-all-together-for-google.html), the quota value in gmail is actually being deprecated. This information is highlighted on the yellow banner of the Google Apps Reporting API (https://developers.google.com/google-apps/reporting/).
So how can you retrieve the quota usage of your account now that only one quota is being applied across three products? You will look up your quota usage using the Drive SDK (https://developers.google.com/drive/v2/reference/about/get) instad. This will give you an idea of how much quota you have used.