I see I may have analyzed the wrong paper -- having now downloaded the links you pointed to, I'll give it a proper study.

For now, however, I'll stand with my original notion that there are more "cost" factors to amortize over the kind of effort Rose discusses than he allows in his discussion.

Adam, of course, makes an excellent point about solar sails (a favourite topic of his, as I recall... G'day, Adam!) -- but again, as with Rose, his discussion limits itself to the actual launch, not the infrastructure needed to get a payload to launch status, nor the mission schedule, nor the survival of the package.

All of those "costs" (that is, their energy equivalent) should be factored into a realistic picture of what such an effort means to the civilization that undertakes it.