Neither club have confirmed the approach and it seems somewhat unlikely that the Wearsiders would think they could get Arca out of the Riverside for such a fee.
The versatile Argentinian's favourite position of all is the left-back berth that Roy Keane is desperate to fill.
But the player himself has not heard of any approach being made and it would take a huge bid to even tempt Boro, where Arca has established himself, not only as a favourite with fans but as one of Gareth Southgate's most highly regarded players among the coaching staff.

Birmingham City 1
Sunderland 1
Stern John proved Roy Keane right when he struck against his former club in the final minute to extend Sunderland's unbeaten Barclays Premier League start at St Andrews.
Keane introduced the former Blues star on the hour having been convinced he was the man to rescue his side, who at the time were trailing to Paul McShane's first-half own goal.
Just like Saturday, when Keane brought on match-winner Michael Chopra, the manager's judgement proved to be spot-on as John nodded home Nyron Nosworthy's cross.
Keane said: "I knew he'd score because in the corridor here I saw a picture of him scoring as a Birmingham player. A lot of players go back to their old club and have a tendency to score.
"It usually happens, so I was confident he would score. I keep saying it is not a coincidence to score so many late goals because the desire and determination was there for all to see."
John bundled home the winner after a goalmouth scramble in which Blues keeper Colin Doyle saved Ross Wallace's free-kick only for Nosworthy to head the ball back across the goal.
John's strike infuriated Blues boss Steve Bruce who felt his keeper had been deliberately held down by Sunderland striker Roy O'Donovan, and that the goal should have been disallowed.
Bruce said: "I was determined to go back in the Premier League and not bleat about referees or linesmen.
"But I've seen their centre-forward jumping on my goalkeeper and stopping him getting up. I think the referee should be seeing that because everyone else is seeing that.
"After he makes the save he's still lying in the back of the net. The referee and linesman have missed it which has cost us dearly and it's a shame because he produced a fantastic save."
Birmingham looked to be on course for their first victory of the season after McShane's unfortunate own goal, when he could only chest Stephen Kelly's header into his own net.
It took until the 75th minute for Sunderland to strike back and they did so in style with Chopra bursting on to Etuhu's long ball and slamming a left-foot strike past Doyle.
But Sunderland celebrations were short-lived. Six minutes later, Blues sub Garry O'Connor cut into the left side of the visitors' box and sent a crisp left-foot shot of his own past Craig Gordon.
But there was more to come from Keane's never-say-die troops. John even had time to head against the bar from point-blank range before he finally found the target.
John had been given generous applause by the home fans when he replaced David Connolly and Bruce put his own disappointment aside to pay tribute to his former front man.
Bruce said: "He was a great servant here and one of my all-time favourites who I have managed here.
"When I saw him warming up and the reception the crowd gave him I feared he might have an impact. But before he got there we believe there was definitely a foul."