There are Rust Belt and Appalachian and New England places where you can get houses for cheap and rent them for a profit... for now. They tend to be places with declining population and no jobs, so in the medium to long term, you're taking a big risk. And you probably should expect to never be able to sell them, let alone see any appreciation.
In decent places that people want to live in the US, there are no "undervalued" houses out there right now.
-W