This campaign has a goal of $1B and Trump has recently demanded $5B for the wall. However, $5B is inly the first installment of the funding. The exact cost is unknown but estimates have steadily increased from Trumps initial estimate of $4B:
September 2015: Then candidate Trump said that while some estimates put the wall’s cost at $10 billion, he could build it for $4 billion.
November 2015: In less than two months time, his estimate went up to $6 billion to $7 billion. (The cost for the wall if someone else were to build it doubled to $20 billion, per Trump.)
February 2016: At the beginning of the month, Trump added another billion to his quote to make it $8 billion. Just a few days later, he upped the estimate to $10 billion to $12 billion.
March 2016: Trump reconsidered, again, putting the cost at $5 billion to $10 billion.
July 2016: By this time, others were jumping into the cost-estimation game. In a widely quoted analyst note, Bernstein Research put it at $15 billion to $25 billion.
January 2017: Senate majority leader Mitch McConnell said it would cost between $12 billion to $15 billion.
February 2017: A leaked report from the Department of Homeland Security put it much higher, at $21.6 billion.
April 2017: On April 18, a group of Democratic staffers at the Senate Committee on Homeland Security and Governmental Affairs published an estimate of $66.9 billion. They came to it by extrapolating per-mile costs based on Trump’s initial request for wall funding, which covers less than 200 miles. On April 21, Trump told the AP that he could build the wall for $10 billion or less.
December 2018: “We’re talking about a wall for $20 billion, $15 billion," Trump said at a Project Safe Neighborhoods conference in Kansas City, Mo. "I could even do it cheaper if I have to, and it will be better than anyone’s ever seen a wall."
The consensus seems to be settling in at about $20B. However, that is the cost estimate, mega projects have a notorious history of cost overruns, and those run by the government even more so. A leading expert on cost overruns is Bent Flyvbjerg, a Danish professor of planning. His co-authored 2003 book, Megaprojects and Risk, concluded that “cost overruns of 50 percent to 100 percent in real terms are common in megaprojects.”. So, the wall could well end up costing something like $30B or more.
Currently the GoFundMe account has raised about $12M in 4 days. At this rate it will take 6,667 days, or over 18 years to raise cost estimate amount. If the project goes over cost then the account will probably have to run for another 9 years.