You could get the same level of detail into a 250 - 300 tri model. There are 2 huge poly waste areas.
1. Arms and legs, 6 sided? Cut that to 4 sided
2. Hair, damn dude I could probably recreate your model using just the hair tri count, get that cleaned up and use alpha maps, if you must have both the inside and outside modeled then simply model the outside, clone it just inside and flip the inner UVs then delete the unseen tris.
When working on this low poly stuff you have to think in tris, not quads. This poly range is an art all its own with its own set of rules.
Heres a rather quick paintover that should help give you the idea with the limbs.