So let's get this straight. You need to buy the materials to build the wall and the tools, vehicles and structures needed to build the wall, you need to pay contractors to build the wall (who you will require to keep paying if you wanna maintain the wall), you have to pay for the roads you will have to make to get your vehicles out there and the eventual maintenance of those roads. You're going to also have to pay every single employee employed on border control, and training them will cost lots of money. After the wall is built and all is said and done, you will need to pay to maintain the wall, which I believe the cost of maintenance will far exceed the original price of building the wall within ten years.
Where the forget is the money gonna come from