First, we haven't. We have hit the point where the amount in the current budget request exceeds the economic damage. The estimated total cost of the wall would be somewhere between $15 billion and $70 billion, depending on who you ask[1].
Second, it still doesn't matter. Giving in to Trump on this matter would only show him that he can expand his racist policies again and again by threatening to shut down the government again. "Once you have paid him the Dane-geld you never get rid of the Dane."
Context is important. For the Republican party, that means decades of using "border security" as a dog whistle for keeping out Mexican immigrants.
For Trump in particular, that means his speeches as candidate, "They're sending people that have lots of problems, and they're bringing those problems with us. They're bringing drugs. They're bringing crime. They're rapists. And some, I assume, are good people." He is a fearmongerer, and this is just another step of playing along fears of immigrants.
Second, it still doesn't matter. Giving in to Trump on this matter would only show him that he can expand his racist policies again and again by threatening to shut down the government again. "Once you have paid him the Dane-geld you never get rid of the Dane."
[1] https://www.politifact.com/california/statements/2017/apr/28...