Tyrrhia: All that totals to $655,370. If we assume as you assumed that 25% of buyers were Russian and spent 70% less, that totals to $655,370 * 0.75 + $655,370 * 0.30 * 0.25 = $540,680.
Finally, by assuming GOG takes 30% of the total revenue, it amounts to $162,205.
That's an interesting exercise and it seems pretty accurate judging by the
CDPR financial reports for Quarter 2 of 2014 and 2015 (the previous summer sales also took place in June). However the number of people who didn't opt in is most likely too large to ignore.
To elabrate, the net profit for 2014 Q2 was $595,547; and for 2015 Q2 it was $1,397,046 (however a large chunk of it is attributed to the release of Witcher 3). So if we assume that the Summer Sale represents 1/4 of Quarter 2's profits (April 1 to June 30) then your number makes sense to estimate the bare minimum of what GOG made this year.
Anyway, we'll know soon enough with the 2016 Q2 report whether you or adaliabooks made the closest guess. :)