This one is inspired by a mechanic in an old game known as Colonization, but it's also inspired by the need for more resource sinks and also to acknowledge the significance of the Armoured Personnel Carrier in modern warfare.
APCs and IFVs are similar in a lot of ways. The only major difference is that the IFV has a bigger gun basically, and the two are often confused. For the purposes of the game, the distinction could be non-existent. Basically they both make infantry more combat effective.
My proposal for APCs/IFVs basically involves increasing the combat effectiveness of infantry in a battle, by basically "mounting" them to a vehicle like Cavalry. Perhaps in a ratio of 1:100 or so. This would stack with munitions if they exist, basically allowing you to do "more with less" in a sense. Unlike Tanks, these units aren't too effective by themselves, being worth maybe 10 or 15 unarmed infantry if equipped with munitions. Unlike Tanks however, they aren't completely useless without munitions since as long as they have fuel, the "mechanized" bonus still exists as they would still serve as protection against small-arms.
For costs, they could either have a separate limit from tanks, or share the limit with tanks. If the latter, this would force you to choose between the expensive Tanks that while powerful, would become useless if they run out of Ammo; or choose the still-expensive-but-cheaper APCs, which would still impose some combat effectiveness if you have fuel but no munitions. Most APCs/IFVs are equipped with either Aluminum Armor or for newer ones, a composite armor. For simplification though, I would make these units cost Aluminum only, as Tanks cost Steel right now. Their monetary cost could probably be slightly cheaper than Tanks as well.
Now for some examples of these units:
BMP-3 (Soviet Union/Russia):
BTR-60 (Soviet Union/Russia):
ATF Dingo (Germany/Various):