id use the battle report in the latest s&p as a guid.
900-1k is a good start, infantry give you a highter break point tanks give you punch so a good mix
for the usmc you have 2 choices as far as i can see
2 M1A2's a box of infantry, the cheapest but an incredibly low break point.
or
1 M1A2 plus 2-3 boxes of USMC's good balance and fun
and dont forget 6 boxes of USMC troopers, near umbreakable but no anto tank ability.