Determining the ROI on PDAs

I am a member of my company’s IT department. In an effort to make a business case for why we should begin supporting PDAs I am hard pressed to determine the ROI on PDAs for a municipality. Our customers are requesting that we support PDAs and I agree they are a useful business tool. However I would like to give more support to my argument then the usual anecdotal fluff. Any suggestions on how I can determine the ROI in this situation?