I think that's a really important question.
There are two points that I think need addressing here.
One, if the Conservative Party of Canada is most interested in cost overruns.... To extend the safe third country agreement to the entirety of the border, we should look to the European Union to see how much it costs to have what the organization they call Frontex comprehensively does with respect to migration controls on their border. The cost overruns would be significantly higher than they are currently.
The second point is whether or not the U.S. is safe. That's a very complicated question. It depends on where you are, who you are, whether or not you live in mixed documented and undocumented households.
I think the way the safe third country agreement is now, the rationale for suspending it is to make it safer for people who don't feel safe in the U.S. to make asylum claims in Canada so that they can enter through regular border crossings. Also, it would address the things that people are worried about, which are the extraneous costs and moving CBSA and RCMP officials around.
It would also remove the kind of spectacular context and images of irregular migration, which are the things that serve as the fodder for populist discourse around unsafe borders and asylum seekers.