Boarder walls are an extremely tight thing. Usually you'll net around 100$ per pop, and one happy point is worth pretty much 1$ income. That means, having 2% less people but 2$ more will, yes, be exactly the same. Now you'll just have to check how much your income is exactly per pop (Taxes only) and how much worth 1point of happyness is for you.
Income 110$, +Happy Point 1$: Don't buy it, you'll loose (exp. 100 pop, income 11000$ before, 10976$ afterwards)
Income 90$, +Happy Point 1.1$: Buy it (exp. 100 pop, income 9000$ before, 9035.6$)
As you can see, in both situations the win/loose factor isn't that big, and most nations tend to have a bigger income than 100$ and will be pretty much on 1$ per happy point, so it usually does not pay off.
Now, why do a lot of nations still have boarder walls?
Easy answer, as long as your environment is not yet 5 stars, boarder walls are worth it in almost all conditions. After you reached 5 stars, stop buying them. And even with your environment under 5 stars, buying everything else will be smarter. Just start buying boarder camps when you bought all other "good" improvments, and that includes stuff like Churches etc.