House Speaker Paul Ryan says President Donald Trump’s border wall will cost $12 billion to $15 billion — and Ryan says Congress will pay for it by this fall.

Congress will move legislation this year providing up to $15 billion to build a wall along the Mexican boundary, Republican leaders said Thursday at their annual strategy retreat. But they would not say how they would prevent the massive project from adding to federal deficits.Ryan said the goal is to complete that and other major bills in 2017, but the leaders offered no details on how the wall would be paid for, saying they would wait until the Trump administration proposes legislation.

WATCH: Trump speaks after orders to build border wall, tighten immigration

Trump is set to speak to the lawmakers later Thursday — a day after signing an executive order calling for the wall.

Trump has repeatedly said Mexico will pay for the wall, but Mexican leaders oppose it and have said they won’t finance it.

Congress will pay for “the construction of the physical barrier on the border,” Ryan said.

“We intend to address the wall issue ourselves,” Senate Majority leader Mitch McConnell, R-Ky., said.

Pressed on whether construction would increase federal deficits, Ryan said Republicans are fiscal conservatives. He said strengthening the economy and replacing President Barack Obama’s health care system were two of the best ways to bolster the government’s budget.

“If we’re going to be spending on things like infrastructure, we’re going to find the fiscal space to pay for that” in a budget Congress plans to write this spring, Ryan said.

Ryan had previously used figures ranging from $8 billion to $14 billion as his estimate for the cost of the wall.

Trump was to speak Thursday to House and Senate GOP lawmakers at their annual policy retreat. Despite a rocky start to his administration, many lawmakers are optimistic about delivering change in a new era of GOP control over Washington.

They would like to see a Trump committed to their agenda and results, not a president who veers off course into conspiracy theories about voter fraud or who keeps litigating the size of his inaugural crowds.

Before Trump’s appearance, Ryan sketched out an ambitious agenda to lawmakers that includes sending Trump a health care repeal bill by March and a rewrite of tax laws by summer’s end.

“I’m just so excited we finally have a chance to do this because we have the House and the Senate and a president who is with us,” Ryan told MSNBC on Wednesday about plans to overhaul the tax system, eliminating what critics say are loopholes and lowering corporate rates to 20 percent or even the 15 percent sought by Trump.

“If you can clean up the cesspool of the tax code and give us a pro-growth tax code, that is how you grow the economy, that is how you take power and money out of Washington and give it back to the people,” he said.

Associated Press writer Errin Haines Whack contributed to this report.