We know that DayZ sold about a million in the 3 months after its release (that is, excluding the first month which had the 'release spike' effect). It had a similar (though still better) profile of steam sales ranking over time. That should set the expectations - about 300k a month or 10k a day is probably an upper bound that is not too far off. So I'd guess about 400k steam sales (inc. EA) and another 150k from retail and GOG.