I never could understand why techers compete with one other to sell res/bus tech @2500 when there's always 0 agri tech.

me too selfie...me too

nOOb effect :-(

a possible reason is maybe because agri tech doesn't do you any good as a techer, while res/bus boost your economy?

This and the agri market generally has less buyers, so it's harder to move on the market than bus/res

Is the income gained per turn from bus/res greater than the added difference between bus/res vs agri?

Has anyone calculated this?

Has to do more with the server dynamics than anything else. If you are playing 360 turns and you're making an extra $500k per turn from teching bus/res, that's $180m right there. If you need the cash, that's the best way to guarantee you get it as who knows when your agri tech is going to sell on this server; the small playerbase means there's only a handful of farmers so if you need cash and they're not on, you're screwed.