The Campaign Finance Institute has issued this report, written by Michael J. Malbin and Sean A. Cain. From the press release announcing the report:

This is the first study in a multi-year effort, The CFI Small Donor Project. The aim of this new project is to understand the role of small donors, large donors and non-donors in state and national elections. The underlying policy goal is to foster democracy by working to increase financial and volunteer participation by the vast majority of people who participate at low levels, or who do not now contribute or volunteer at all.
The current study focuses on the relative importance of small and large contributions to federal political parties and candidates in the years before and after the 2002 Bipartisan Campaign Reform Act (BCRA).