Um, what? You do realize that roads have positive externalities on the rest of the economy, right? Goods purchased everywhere end up being cheaper if the cost of moving them and their component parts around the country is lower. Fences don't do that.
And have you got any kind of source for that cost estimate? A city like, say, Houston, employs 5400 cops, and has a budget of about $650M/year. Apply that to the border, and you have 2 3/4 people per mile. That would certainly catch some people, but coverage would be well short of 100%.
Then, you've got to consider the other half of the equation-- what would be a fair dollar-figure estimate of the benefits of "securing the border"? (Accounting for the costs, too, like how much more expensive domestically grown produce will be when the industry is forced into using a wholly legal workforce. And construction. And childcare. And on and on. The prevalence of the illegal workforce is a bad thing for a lot of reasons, but getting rid of it does mean that costs for a lot of things are going go up.)
2 3/4 people per mile is plenty to patrol a fence. I think LA has something like 15000 cops. The benefits would be enormous, we would dramatically cut crime, drug smuggling, and illegal immigrant social care costs. Plus we wouldn't lose our country, like every other dumb**** in history.
Edit: And your argument is what, we should take advantage of the cheap illegal labor, while hypocritically passing minimum wage laws?