The roof will remain open on Saturday, but Australia wanted it closed
Three of Wales' autumn matches have unsold tickets, says the Welsh Rugby Union - with more than 21,000 spare seats on Saturday against Australia.
Only the 27 November fixture against New Zealand is sold out, with South Africa and Fiji still having capacity.
Wales coach Warren Gatland acknowledged on Thursday fans might find the price challenging as he thanked those who had already paid between £25 and £70.
"It's tough times out there for everyone at the moment," Gatland said.
"Those fans who have put their hands in their pockets to support Wales have made a real commitment to the cause already," he added.
"We have spoken this week about the responsibility we have to those fans now to perform."
Cardiff's Millennium Stadium has a 74,500 capacity - but on Friday morning, ticket sales for the first match against Australia stood at around 53,000.
The Welsh Rugby Union released a statement which said supporters would be able to buy tickets on Friday and Saturday from their outlets in the capital's Westgate Street, and promoting availability for the matches against South Africa on 13 November and Fiji on 19 November.
Players themselves are aware of the challenges both on and off the field, with scrum-half Mike Phillips promising high-tempo, attacking rugby to counter Australia's "dangerous" game.
"We want to play a quick game as well - we want to be entertaining," Phillips said.
Meanwhile Saturday's match will be played under an open roof - although Australia had wanted it closed in bad weather to aid their expansive tactics.