UIUC Extension resources are your friend. 

http://wq.illinois.edu/DG/DrainageGuide.html

All sorts of resources there for sizing mains and laterals, determining spacing based on soil type, installation depth, and desired drainage coefficient, and calculating ROI, IRR, etc (as always, based on your assumed yield increase and per-bushel sales prices)

At your planned 40-ft spacing, you are looking at (43560 sq ft/40 ft spacing =) 1089 ft of lateral per acre. 

Cost per foot seems to vary wildly according to local supply and demand - the actual 4" tile material here is like $0.30-$0.45/ft (depends on oil price and tile demand).  Installed 4" tile cost here (tile + installation) around $1/ft.  I have seen total 4" costs in N IL approach $1.40/ft.  The guys in MI talk about having tile put in for $0.20/ft install cost, on top of the actual tile cost, on their close spacings (15-30 ft spacing).

If you have a good outlet running through your farm, you probably don't need any expensive 8" tile - just a series of 6" mains running to the outlet, and start a new main when the original main is filled, which will depend on your ground slope.  Flat is a very general concept that means very different things in different parts of the world.