For what it's worth (and I think most people would agree with me), politics aside, I am not for or against walls in any fundamental way. To know if a place that doesn't have a wall needs a wall, I would have to know how many and what kinds of people are crossing, who owns that land, what the cost of building a wall there would be, what other options might be available and what those would cost, what the costs of manning that section of wall would be compared to what costs might be currently spent in that area, what the environmental impact would be expected to be, and how easily the border crossing traffic would just be relocated somewhere else if that section of wall was built. And all those things would vary from spot to spot.
None of us know all of that with enough depth to know if $1 B or $5 B or $20 B is the amount of wall that aligns with our personal beliefs about how hard we work to prevent border crossing.
This argument is about whether or not Trump gets to deliver on a part of his campaign promise. If he was serious about it, he should have acted before the Democrats took the House. If the Dems hated walls, they would have fought more against them in the past. This is all political theater.