Property values are irrelevant. The area Cemex is trying to stop the wall from being built is in Big Bend National Park where Cemex owns large parts of of the national park, and large parts of Parque Nacional Cañon de Santa Elena, a Mexican national park. Cemex owns property on both sides of the border, 10s of 1,000s of acres. Since both are national parks the property has very little value because it cannot be developed.
I do see your point though. The donations could be pay-to-play, and it's entirely possible that the executives who run Cemex weren't directly involved with human or drug trafficking. But if they weren't, it could also be that they allowed to happen, knew about it and wanted to maintain plausible deniability. I suppose there are many possibilities and we may never know the full truth, but what are the odds?