Let's do some guesswork how much a DLC needs to cost: I guess a sensible price for an 'Interiors DLC' would be around 30-35€. Let's multiply that with 5-8k early buyers, then you get the potential revenue as a basis to finance the project. The margin is probably a good 25% lower (optimistically).
35 euro times 8000 purchases at 25% overheads is about £250,000 in UK spending, which would probably pay for a team of four for one year (including equipment costs, non-cash compensation, etc). On the scale of ED's income and expenditure it's not quite a rounding error, but it's still pretty small (about equivalent to two week's ED income over the last year, so probably not
that far off two week's expenditure either)
Odyssey - which was a massive failure financially, of course - raised about £7 million on the strength of pre-orders and early sales, looking at Frontier's investor presentations. They then wrote off a further £7 million loss on it a year later, so it must have cost at least £14 million and maybe a bit more to develop. (And these are in 2021 £s, so add a bit more on again for inflation to get 2025-equivalent costs)
So a DLC of Odyssey scale, sold at something like Odyssey's original £35 price, would need something like 400,000 sales to break even (and it got maybe half that in its first year, almost all in the first month). To get it to work with a mere 8,000 sales those 8,000 would then need to spend about £1,750 each on ARX extras for it.
Obviously there's room to decide that ship interiors would be a smaller expansion than Odyssey and so only cost e.g. half as much to develop, put the basic price up a bit to allow for inflation, guess sales numbers larger than 8,000 but smaller than 400,000 that require vaguely plausible ARX spends, etc. I'm sure those who want to hope can find a set of numbers which they can believe.