Yes, during the sales process your SAP point of contact can provide a match analysis to verify the number of your suppliers that are already using the Business Network, and it is likely that many of them already are. SAP Ariba does not charge suppliers fees to participate in the Business Network, you can read more about it
here. The success of supplier adoption is more closely tied to the enablement plan designed by your implementation partner. At Optis, we have regularly achieved >90% supplier adoption across direct and indirect spend.
This is the number one contributor to realizing the benefits outlined in your business case and is also the number one long term failure point of source-to-pay programs. The approach to ensuring success is having a comprehensive enablement program that includes collaboration in the project across catalogs, purchase orders, invoices, and engagement methods (email, supplier portal, and cXML). Most importantly, after go-live, full supplier adoption either requires transition to dedicated client resources, or ongoing "white glove" support from your implementation partner.
Many market analysts release annual reports on their view of the market, pulling from their own intelligence sources. These analysts include Forrester, Gartner, Spend Matters, and many more. Beyond market intelligence, consulting firms which implement the solutions have hands on experience with the technology and their customer base and can give you a better understanding of how your organization might be suited to the solutions on the market. Optis has experience implementing for clients in virtually all industries and can provide a tailored perspective.