See: "A Blunted Sickle".
Steven Eden in "Military Blunders II," estimates that had the French instead spent those resources on tanks or planes they could have bought an additional 24 tank divisions, or 10,000 fighter planes. Obviously the Maginot Line was built over the course of a decade, so that by no means would have allowed France to actually enter WWII with an additional 24 tank divisions or 10,000 modern fighters - but had the French military been spending those resources developing its inter-war maneuver forces it would have had a much stronger foundation beneath it in 1940.
I wonder how he calculates this. Randal Reed in a magazine article from the late 1960s provides this:
Cost of Units Vs Cost of Maginot Line
Units..................Cost of Unit in............No. of Units Available
.........................F 1,000,000.............for expense of Maginot Line
.................................................. per mile.............for whole
Motorize One
Division (cost of vehicles)
100% trucks.........78.75....................1.02...... ...........86.74
80% trucks..........115.5.....................0.70.... .............60.9
20% tracked
Divisions
Infantry...............175.......................0 .46................40.02
Armor.................280.......................0. 29................25.23
Aircraft
Fighters................0.7....................115 .1..............10,013.1
Bombers Med......2.187.....................36.8........... ....3,201.1
Bombers Hvy......3.937.....................20.5........... ....1,783.5
Reed goes on to point out French industry did not have the capacity to produce this many trucks, tanks, or aircraft; and it had limits on expansion. This was not simply factory floor space, machine tools, but clear limits on skilled labor. He goes on to refine his numbers to:
Motorized Infantry Divisions............+45...total 60
Armored Divisions.........................+10...total 17
Fighter Planes..........................+1,000...total 1,700 (modern types)
Bombers...................................+500...total 1,225 (modern types)
As part of his refinement he allows for cost of training, and support costs. that is the cost of adding a fighter pane is not just the cost of a air frame and a couple spare engines & other parts, but also the mechanics, riggers, fuel handlers, clerks, ect...
This cost of support crew then touches on limits of people. There only so many able bodied men, or women to allocate & France like most other nations could only increase one group by decreasing another. There was not a huge surplus labor force as there was in North America.
Anyway, that is Reeds estimate. One thing that some folks might catch is only sexy items like tanks or aircraft are calculated. Reed did not do estimates of items like extra corps artillery, AT guns, artillery ammunition.