The free agent, who recently left West Ham after rejecting their latest contract offer, is mulling over offers from several clubs including Atletico, Galatasaray, Sunderland, Blackburn Rovers and Hull City.
With the new tax rate of 50 per cent for Britain's high income earners set to be implemented next April, Neill is seriously considering a switch to Spain after 14 years in England.
"I am in talks with a club outside of England," Neill told the Australian media.
"That's been the hardest one because of the language barrier. It's a change of what I've been used to.
"A decision will take a lot of courage so that's why I've taken my time."
Atletico's offer of around £1.5million-a-year to the 31-year-old is less than what West Ham offered, but Neill will pocket more in Spain due to the country's lower tax rates.