A fence is useless anyway. We need a wall like the vatican or Israel..the one Trump is talking about. I think there is more to getting Mexico to pay than just using Trade. Look at the companies that are moving to Mexico, so perhaps lowering the corp taxes/etc. for the companies will halt more companies from moving there, and perhaps bring some back to the US. Since we have a wimpy useless spineless Congress, maybe Trump can do an executive action to get it built.
The border between India and Pakistan is a good example of what can be done if you have the political will to do something rather then just talk.