I think it would be quite appropriate for it to go to some form of border security, considering the amount of drugs El Chapo was responsible for running across the southern border, as well as elsewhere. As far as an actual physical barrier, and all of that money going to pay for one, I'm not sure if that's the best thing, considering the different situations you run in down there with wall building. But certainly, the money could be used for helping to better secure the border and drug enforcement.
Whether it happens or not, I'm not holding my breath. Even if the U.S. does get the money (and I'm sure there are other countries interested in it, including Mexico), the government has a bad habit of finding other ways to squander money it brings in.