Not sure why you're trying to overcomplicate what is an attempt to create a simple ballpark figure, but let's work on the following assumptions:
a) We have 5,000 season ticket holders on the Kop, it may well be more but let's call it that for simplicity
b) To reflect the improvement in facilities from 30 year old shithole to brand new stand, we can justifiably increase Kop pricing up to what it is in the Westfield/BLUT, I don't have exact figures on what a Kop ST is to hand, but calling it an extra £50/year doesn't seem too far off what the difference is
c) Increasing overall ground capacity by, say, 3,000 (as quoted in previous proposed plans, could theoretically be more but let's use that as a number) would allow us to increase the number of season tickets we can sell by the same. I would imagine this would actually be higher, as I would guess we are keeping more than we need to for pay on the gate that we need to (because nobody in their right mind would buy a season ticket in either shithole corner or with a restricted view, problems which would both go away with a new stand), but that's not necessarily important given the below
d) While we are currently selling out season tickets, both in and out of the Premier League, I would expect increased ST capacity to sell out, but I'll play devil's advocate and say we only actually sell an extra 2,000 of the increased capacity
e) Will use £450 as the new Kop season ticket price. I paid slightly cheaper in Westfield (see b), but that was both a renewal and in first wave, at any other time it is more so call it an average figure and/or factor in inflation by the time the thing actually opens
So for existing let's call it 5,000 x £50, for new let's call it 2,000 x £450, that's £1.15 million a season. Will round it down to £1m to account for using adult pricing given not everyone in the Kop is paying full rate (although I think it's probably more in the Kop percentage wise than other stands). This is using conservative figures for what the current count of season ticket holders is, expected future demand, and completely ignoring pay on the gate support and all other ancillary revenue.
We may differ on what we perceive the current and potential size of our fanbase to be, but I don't think anything I'm saying there is either unreasonable or overly optimistic (if anything, I'd say I'm being excessively cautious)?