Anyone, even the diehards at Parkhead, would take one look at that figure and snarl at the notion of the rejuvenated midfielder leaving the club for such a fee.
The £20 million, of course, is the buzz figure at the moment with striker Moussa Dembélé being linked to West Ham for that amount.
Nevertheless, Petrov has indeed highlighted Armstrong's quick progression as a box-to-box midfielder under new manager Brendan Rodgers.
Purchased at the end of the 2015 winter transfer window, alongside best friend Gary Mackay-Steven from Dundee United, Armstrong excelled with his chum to become very influential in their early Celtic careers. Armstrong was one of the key components in Celtic's dramatic three all draw against Internazionale at Parkhead with the Aberdeen-born midfielder netting one and heavily influencing another goal.
Under former boss Ronny Deila, Armstrong seemed a frustrated player which has been subsequently revealed as a result of being played out of position. At Dundee United, Armstrong was a central midfield player and received plaudits from across the country and also managed to net against The Bhoys at Parkhead before joining later that season.
However, Deila would consistently play Armstrong wide left of three attacking midfielders behind the striker and it was telling that he was never comfortable there. Fans would often slate Deila for this decision, often calling it baffling, however, a sensible head could see that Kris Commons and latterly Tom Rogic were always going to occupy that central spot with good reason.
Commons had just come off the scoring season of his life and was still banging them in, only to be taken over by another improved player in Rogic who has made that position his own ever since.
After a great start at Celtic, Armstrong's performance level dipped and many weaknesses crept into his game such as misplacing passes, and a lack of confidence meant beating a man one-on-one was always a struggle.
The Brendan Rodgers effect
Fortunately, with the arrival of Brendan Rodgers, everyone is now seeing the reformed Stuart Armstrong much to the delight of the Celtic faithful.
Many will tell you that Armstrong would probably have made their list of players to ship out in the summer window - there didn't seem to be any place for him as Celtic had amassed an abundance of average midfielders. Rodgers' man-management and stature appears to have worked wonders not just on Armstrong but the entire team as they continue unbeaten domestically into the new year.
The 24-year-old midfielder has netted eight goals and provided five assists this season from his new, but familiar, central position alongside captain Scott Brown. From the start of the season, more and more fans remarked 'Stuart Armstrong played well today'. Week after week, this became the norm until it was accepted that a good performance was always going to come from him.
Armstrong cemented his return to good form by netting against Rangers in a 5-1 demolition derby at Parkhead in September, however his performance against Motherwell at Fir Park was also one to savour.
Armstrong has also continued what he has always said - that he'd play anywhere to get into the team.
Often he has found himself back at his old position under Deila and also made a token appearance at full-back after which Rodgers dubbed him the 'Blonde Cafu'.
There seems to be a burning desire with Stuart Armstrong; lung-bursting runs forward, tracking back to slide tackle, as well as becoming a midfield anchor - these were not traits that could be said of him at the end of last season.
Petrov may have stretched the boat out ever so slightly to say that he could go for £20 million when his journey with Celtic comes to an end, however, it just shows how high in regard he is with players who have seen it and done it at the top level with the club.
Future international anchorman?
Ex-Celt and Dundee manager Paul Hartley has called upon his old manager at Parkhead Gordon Strachan to consider selecting Armstrong for the national side - who are underperforming in their group for the World Cup.
Scotland's national team lack pace in the central area and a fast player in that position with a high work rate could help in tracking back to help the slow centre-backs currently deployed.
A change in mentality for the Scottish national team is necessary, and fast, as an extension of exile from yet another competition looks likely.
For club and country, Stuart Armstrong could well be that missing link that both have been missing for several years and under Brendan Rodgers' wing, his talent could be nurtured to become the next leader for Celtic and Scotland.