This is probably one of the relevant documents. From the DFG website:

http://www.nrm.dfg.ca.gov/FileHandler.ashx?DocumentID=3871

It includes a summary that Rob would probably enjoy:

"Published estimates of the costs of the American zebra mussel invasion vary greatly, and the actual cost remains uncertain.16 Extrapolating from surveys conducted in 1995 of a portion of affected facilities, the retrofitting, operations and maintenance costs to facilities in eastern North America appears to be somewhere around $100 million per year—not including secondary economic costs or environmental costs."

And the lengthy footnote:

"16 For example, the U.S. Congressional Office of Technology Assessment projected U.S. costs of $3.4 billion in 1991 dollars over 10 years (OTA 1993), or somewhere around $550 million per year in 2007 dollars. An often cited figure of $5 billion—given as $5 billion in the U.S. through 2000 by Miller et al. 1992, as $5 billion in the Great Lakes through 2000 by Ludyanskiy et al. 1993, and as $1-5 billion annually in the U.S. by Aldridge et al. 2006—is apparently based on a projected cost of $4.82 billion in North America over 10 years, of which $2.11 billion was for impacts to facilities and vessels and $2.71 billion was for impacts to Great Lakes fisheries (C.R. O'Neill, pers. comm.). Other published figures include a projection of $2 billion in the Great Lakes region over 10 years (McMahon et al. 1993), and in two frequently cited reviews of the costs of invasions in the U.S., estimates of $100 million per year (Pimental et al. 2000) and $1 billion per year (Pimental et al. 2005). In most cases it's not clear what these estimates and projections are based on, and whether they are limited to facilities costs or include secondary or environmental costs.
"

So, yes, the economic costs are uncertain at best. But my point still stands, which is that the water management agencies are making their decisions about requiring boat inspections based on their best estimates on how their decision will effect their budgets.