Sainz must leave Ferrari at the end of the season to make way for Mercedes' seven-time world champion Lewis Hamilton.
The Spaniard, race winner in Australia last month, told reporters at the Chinese Grand Prix on Thursday that all his best options remained open and it would take some time to reach a decision.
Sainz has been linked to Red Bull as a possible teammate for triple world champion Max Verstappen, recreating their 2015-16 Toro Rosso lineup, if Mexican Sergio Perez does not have his contract renewed.
Marko told Austria's Kleine Zeitung newspaper that Sainz was of interest to Red Bull but that Audi, who will be taking over backmarkers Sauber in 2026, have offered him more than Verstappen's team can.
"We're talking to him, he's having his strongest season in Formula 1," said the Austrian. "But he has a very lucrative offer from Audi that we can't match or beat."
Marko said the experienced Perez was also having his best season since joining them in 2021 and will be the team's first choice for the second 2025 seat if he keeps performing as well as he is.
"If he maintains these performances like those in qualifying in Japan, then he is certainly the best option for 2025 at Red Bull," he added.
"He is a worker for the team and has now also realised that the radically different path he took last year with setup was the wrong one. Now the car is closer to Max's setup and that helps him."
Australian Daniel Ricciardo has also been in the frame for a possible return to the main Red Bull team but has yet to assert himself at the renamed RB, the outfit previously known as AlphaTauri and Toro Rosso.
Marko said that so far Yuki Tsunoda had Ricciardo "under control".
"The challenge for him (Ricciardo) was that he clearly had to be faster than Yuki if he wanted to have any hope of winning the seat at Red Bull. That hasn't been the case so far, even if, as I said, it was close. We'll see how this develops," he said.
