Monbvol and I have been working through those exact numbers. The minimum ranks for a MechWarrior are either E-5 (AToW, page 124) or "O-0" (as outlined in the same table, and explained in the AToW Companion, page 28). I like to think of O-0 as "Warrant Officer". The monthly salary difference (per the Campaign Operations formula on page 25) is:
MechWarriors:
E-5: (5/3+1) x 1,500 = 4,000 C-Bills per month
O-0: (3/3+1) x 1.2 x 1,500 = 3,600 C-Bills per month
and for comparison:
O-1: (4/3+1) x 1.2 x 1,500 = 4,200 C-Bills per month
Vehicle Crew have a lower base, and could be lower ranked as well:
Vehicle Commander (E-5 per the table on page 24 of the Companion):
E-5: (5/3+1) x 900 = 2,400
Vehicle Crew (charitably interpreting the same table in the Companion, which says an E-2 can be a "Trooper"):
E-2: (2/3+1) x 900 = 1,500
And a Vehicle Lance Commander for comparison:
O-1: (4/3+1) x 1.2 x 900 = 2,520
So, a vehicle with two crew would cost 3,900 to man, while one with three crew would be 5,400, and four would be 6,900.
Infantry also has a lower base, but again, greater numbers:
Squad Leaders are E-5 as outlined in both AToW and the Companion tables:
E-5: (5/3+1) x 750 = 2,000
Troopers are E-2s as discussed above:
E-2: (2/3+1) x 750 = 1,250
But in a generic squad there are six troopers, so a squad would cost 9,500 in salaries.
Every four squads requires an officer:
O-1: (4/3+1) x 1.2 x 750 = 2,100
Now, here's the real trick. Every 'mech and vehicle requires a tech team (1 Technician, 6 AsTechs, using the CampOps table):
Technicians have no guidance on rank, but I think E-4 is reasonable for a "Specialist":
E-4: (4/3+1) x 800 = 1,866
AsTechs also have no guidance, but like the Vehicle Crew, "Trooper" seems appropriate:
E-2: (2/3+1) x 400 = 666
So a 'mech Tech Team adds another 5,866 C-Bills per month in salaries. Things get more complicated if you use the AToW table referenced by CampOps (AToW page 335 differentiates the various kinds of technician, but doesn't have a separate entry for AsTech, instead relying on a "Quality/Expertise" multiplier). Infantry needs one Tech Team per four Platoons (CampOps, page 20).