The average number of articles published per journal per year is ~110. Let's say a major journal publishes probably closer to 300/yr.
Assuming you try and barebones it with 3 staff members, a technical lead for screening, a graphics / visual editor, and a peer review manager. Assume you want someone relatively competent for your journal so you pay each (inclusive of overhead & benefits) ~$150k/yr.
$150k/yr × 3 / 300 articles = $1.5k/article
Again, not saying it's a perfect system and things can definitely benefit from economies of scale, but it really doesn't take much to get $1k/article in expenses to pile up.