True, true... I've been looking at it purely from support costs, not initial investment. The trick there is that a MechWarrior's salary is only equal to two infantry troopers, but the one MechWarrior can bring many times the firepower. Vehicles fall somewhere in between (unless they only have a crew of one, in which case they act as really cheap 'mechs).
The support cost for even a 100 ton 'mech is only 1,000 C-Bills per month, but one way to partially level costs would be to hold the line on tech team requirements. As we worked out before, a full (regular) tech team costs 3,200 per month (800 for the Tech, 2,400 for six AsTechs). Requiring one tech team per 'mech, one per lance of light vehicles, one per two heavy vehicles, and just a Tech per company of infantry (the troops can act as their own AsTechs) should level things out a bit. Even with that though, a single 'mech is going to come out to less than the salary cost of a single squad of infantry (5,250).
This will definitely require more thought...