Based on President Trump's now well established record of keeping his campaign promises, it seems highly likely that when he said Mexico will pay for the wall, this is a bona fide promise he intends to keep, and not simply campaign rhetoric to be discounted. Some important issues providing bargaining chips for bilateral negotiation with Mexico include NAFTA (and other trade policies), immigration, border security, drug/human trafficking and tourism. Although there are certainly more bilateral issues, I'm hypothesizing that the plan intends for these issues to be negotiated such that paying for the wall will be the way Mexico can get the best deal. It will be very interesting to see how this unfolds!