I think that's because coming up with the number is really hard. We're starting—and I say "we” as the royal we—with limited funding and, with something like fighting invasive species, particularly on a national scale, you're talking about a large dollar figure to do it right, the way that we'd like to see it done.
What it really comes down to is investing, no matter what, in the risk assessments to know what you need to tackle first, and then that will give an indication of how much money you need to spend.