Wall says it's inappropriate for an elected official to accept a fee to talk at such an event, so Trudeau should pay back the money.
"I just think in terms of an example of leadership that that's the right thing to do at this point," Wall said Friday.
"He's now an aspirant to be the prime minister of the country. I think it's wrong for MPs or MLAs, for those to elected office, to take money for speeches that we ought to be giving because we're already paid our wage and so, because these are charities in the main, I think an offer of reimbursement is the right thing to do."
The Saskatchewan premier says the conference, held in April 2012, was left with only a $7,000 surplus for its next event.
The issue came to light after Wall was asked about speaking fees in an interview with Global TV.
In the TV interview, Wall noted that the speech came just before the Quebec MP decided to run for the Liberal leadership and he questioned whether the money was used to help finance Trudeau's leadership campaign.
Wall later said in a statement that if Trudeau says none of the money he charged to charities aided his leadership campaign, "then I accept him at his word." However, the premier also said he makes "no apologies for asking questions of accountability."
"Clearly, it's not $20,000 to fly from Ottawa to Saskatoon, so this is about revenue," said Wall.
"I've heard his argument...that there's lots of politicians, MPs for example, as private members who have other careers, some continue to practice in law, maybe some continue in some other vocation, and make money.
"But really this goes to the heart of what we do in public life ... and I think it's just fundamentally poor judgement for somebody who clearly at that time was going to be applying for the job of prime minister, to charge $20,000 to do what I believe we as MPs and legislators, MLAs in this case, are paid to do already."
Liberal House Leader Dominic LeBlanc took issue with Wall's comments.
"Premier Wall needs to immediately apologize for this smear. It looks like the race for (Prime Minister Stephen) Harper's job is on," LeBlanc said in a statement.
"Trudeau's campaign was able to raise a significant amount of money during the campaign, so much so that at the close of the campaign they were able to transfer over a million dollars to the national party. Every donation made was a personal donation in compliance with Elections Canada laws and was fully disclosed."
The Liberals said Trudeau's chief of staff has contacted Wall's office "and we are expecting an immediate apology."
It's not the first time Trudeau has faced heat over speaking fees.
In February, Liberal leadership hopeful Martin Cauchon questioned Trudeau's judgement in continuing to collect public speaking fees while serving as an MP. Cauchon said speaking to charities and other non-profit entities should be part of an MP's regular duties.
Cauchon noted that it's not illegal, but he said it was a mistake for Trudeau to accept speaking fees from not-for-profit groups and Cauchon called on Trudeau to reimburse the money.
LeBlanc pointed out in February that all of Trudeau's speaking engagements were cleared by the independent parliamentary ethics watchdog.
Wall's comments came amid media reports that Trudeau has turned down a request to give back his speaking fee to a New Brunswick charity that was trying to raise funds for a seniors home.
"The fundraising event we hired you as a speaker for was a huge disappointment and financial loss for our organization. We are a new foundation attempting to raise badly needed funds for the elderly residents of The Church of St. John and St. Stephen Home Inc.," wrote Grace Foundation board member Susan Buck.
"A refund of the fees charged for your speaking engagement to the Grace Foundation would meet our needs and would provide a positive public impression."
The matter was also brought up in Parliament during question period on Friday, where Immigration Minister Jason Kenney said politicians usually attend charity events to try and help raise money.
"We regard it as part of our job," he said. "It’s frankly unthinkable to me that a member of Parliament getting paid $160,000 a year by taxpayers would go and take $20,000 from a seniors’ charity. It just suggests terribly bad judgment."
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