I dunno about cheaper, but they should be available from 1936, not 1940. All the European majors had bren carriers and halftracks at the start of the war.
Mech is not only about the vehicle hardware, the halftracks or full tracks. One of the smart design ideas in HOI IV is that new tech development encompasses what in reality is a combination of the hardware and development of the tactical doctrine and methodology how it is used. This is further extended in the operational and strategy level doctrine trees.
To produce mech units is about developing both the vehicles and a completely new unit approach for blitz warfare. With the emergence of mech units, combined arms of tanks and infantry could fire and attack while moving. This was a new paradigm of warfare and it should cost to develop, as it cost a lot of time and methodology development in reality. In the game a well balanced armoured division is an awesome combat unit, just as it should be.
The German general Guderian "invented" blitzkrieg tactics in the1930s, influenced by the British military theorist Fuller. But Guderians blitzkrieg was combined arms warfare with tanks supported by Stuka dive bombers and fighter planes. The role of infantry was to mop up any resistance left after the tanks had passed. But this was a very costly approach to offensive, as the tanks were easily defeated by small infantry units with mobile anti-tank weapons.
Only from approx 1943, full combined arms armoured warfare was developed and tried in the Kursk battle. It did not work well in the Russian prepared Kursk defences, but that is not the point. The point is that the tactics and ability for armour - mech offensive action was invented there and then, and then well utilised by Patton in France in 1944-1945, and thereafter by the frozen armies of the cold war from 1946 to 1989. Combined arms doctrine did not change much after 1943, with the exception of new armoured helicopter units from the early 1970s.
Real combined arms action is not possible without BOTH tanks and infantry in armoured fighting vehicles, where infantry can fire while the vehicle moves. So yes, it does and should cost to produce such units.