No your maths is fine. My point is that you make more money in the long run with the corners.
If you take it from a "per seat" view...
Both corners and endzone, cost per seat to construct is:
$543.48
If you sell a ticket for a corner you make $38, sell one for an endzone and you make $32.
In comparison, look at the difference between this and expanding your current sideline seating.
Cost to construct 1 seat = $1,000. Ticket Sells for $42. Doesn't make sense does it?
I made the mistake in my first year expanding my current seating... big mistake.
It only become profitable when you get to building the level 2 sideline.
The cost per seat of building level 2 sideline is around 750 dollars, something like that. but you only make $32 per ticket. So once you have built level 2 sidelines, it actually makes sense to expand the level 1 sidelines.
There's a really good spreadsheet
http://spreadsheets.google.com/pub?key=pTLj2bA3Ha6tRZr5ZNaLtAw