This report by the Center on Budget and Policy Priorities has estimated that overall compliance costs are around 7%. This includes IRS expenses and taxpayer costs for both planning and administration. Here's the relevant part of the report:
The public and private costs associated with estate tax compliance -- including IRS costs to administer the tax and taxpayer costs for estate planning and administering an estate when a person dies -- equaled about 7 percent of estate tax revenues in 1999.
[21] That is within the range of compliance costs for other taxes. For instance, administrative and compliance costs equal about 14.5 percent of the revenue raised by the individual and corporate income taxes and about 2 to 5 percent of the revenue raised by sales taxes. In addition, the number of individuals and estates that bear these costs has fallen markedly as the estate-tax exemption level has risen since 2001.
Facts sure are pesky things, aren't they?