Need Help Selling the DISC Profile System to my site

Have you considered starting with a small group to demonstrate success prior to requesting the entire site?  If you can "sell" the request for a smaller team and measure the results over a  period of time you may be able to obtain approval for a larger effort.  In terms of measuring success I am thinking some sort of organizational culture survey or something similar.  If your company already does one then you can look at how communication and teamwork have changed over time.  If your company does not do one then maybe you can initiate a project to measure and increase teamwork, communication, etc. starting with a small survey in advance will set the baseline.  Over a period of time leveraging the DISC system (pointing out better communication styles in O3s, regular meetings, etc.) will increase your metrics.  I would imagine that an increase in the OCS will more than justify the modest cost of the system to the beancounters.
I hope this helped.
Greg

About 10 years ago, when I was responsible for Learning & Development in my organization, I started an executive coaching program for mid-managers. The first step in that program was to take the DISC profile. The managers loved both the coaching and the profile. So, if you can't convince HR to do it as a standalone program, then perhaps you can convince them to bundle it with something complimentary.
Actually it wasn't my idea, it was the coach's so if you can gain a recommendation from the vendor, then perhaps that will add to the credibility, especially since the vendor will most likely be from the HR profession and be able to answer your HR department's objections.