Isn’t the base game with the story / single player is like 3$ on pc ?
The pricing discounts over the year definitely have helped with the sales numbers...
However I think it'd be more interesting to know what total revenue SFV has generated. In that area, I think SFII still has it beat; remember SFII was a TITAN in arcades for a very long time, between the original and the other variants. Especially if you adjusted for the modern cost of inflation, I kind of doubt SFV has surpassed it in total revenue.
That said, I think it's easily surpassed SFIV and its variants in total revenue, which should be all the more proof that SFV being PS4 console-exclusive was NOT the reason it struggled early on. Such was always a ridiculous assertion, when the proof of the game's poor release state, between a gimped cast, tons of missing content, and horrible netcode, were right in everyone's faces.
If SFV launched in a more competent state, it'd of probably sold closer to 7-8 million LTD, without need for as many pricing discounts or sales promotional prices too, and would have generated even more revenue. But its results amid where it started to what it's managed to turn into now, are no less impressive.