President Trump’s chief budget official told lawmakers on Wednesday that the administration asked for more than $4 billion for border defense, with half of the funding dedicated to “the wall.”

In a hearing before the House Budget Committee, Office of Management and Budget Director Mick Mulvaney answered questions about Trump’s paramount campaign pledge -- construction of the border wall along the U.S.-Mexico border.

Rep. Glenn Grothman, R-Wis., asked how much of the wall would be completed by the end of the fiscal year.

“It’s really difficult to say -- we are going through a prototype process and we’ve not decided if one size fits all,” Mulvaney said, and told lawmakers the administration will determine the type of wall necessary for each part of the border.

Grothman asked how many miles the border wall would be.

“It depends on the kind of wall -- a bollard wall is $8 million a mile,” Mulvaney said.

According to Mulvaney’s prepared opening statement, which he opted out of reading to the committee to get straight to questioning, the president’s budget makes “historic investments” in border security and immigration enforcement.

The budget provides more than $2.6 billion in new infrastructure and technology investments to give Border Patrol agents “the tools they need.” The budget invests $1.6 billion for new and replacement border wall in “priority areas.”

But Mulvaney assured lawmakers: “Work is going on, on the Southern border, today."

Fox News' Jennifer Bowman contributed to this report.