These guys are saying their costs are $4M per mile, which who even knows if that's true, and that they already have other property owners and other segments lined up. They apparently think there's more donation money out there that might come in if/when they can demonstrate some results.
So if their fence actually does divert the traffic to other non-fenced properties, owned by other property owners, then over time that could change the math on which nearby property owners choose to participate.
If these guys know what's good for them they will go to extreme lengths to account for every dime of the donation money because the no-borders contingent will be scouring the situation in search of any corruption. If these guys are dumb about it then they get what they get.