Haha 8 billion is a good joke. As projects scale in size the costs scale astronomically. Also, comparing the effectiveness of a small wall which can easily be manned to a massive 1000+ mile border is ridiculous. We'd need tens of thousands to watch the border for what is a relatively minimal threat to our country.
Shutting down relations with a neighboring country via sanctions is a good way to permanently damage relationships. Just look at what economic sanctions in the past have done to US relations with Cuba, Iran, etc. It is a grave move to make and isn't one that should be taken lightly, especially in a time when there is a net negative migrants from Mexico to the US.