Those are both fair points. I would counter, however, that having a wall will help keep out illegal aliens who are presently in the US and get deported in the future from being able to return.
Thus, the effect of the wall may not be merely to reduce an influx of new illegal aliens from arriving, but it may actually enable us to reduce our present population of aliens. Plus, we have omitted the savings on border patrol, which one would anticipate will be greatly reduced.
The effectiveness of the wall is a little too speculative to enable a prediction model. A fair middle ground is to say that even if the wall shaves the influx in half, and costs $2B a year to maintain, it pays for itself fairly quickly.