Each model have a different difficult to create, varying by the level of quality you need and the complexity of model. Because this if you don't have a very detailed specification of what you need the negotiation whit the freelancer can be very long.
- Try make a list of references pics of quality you need (samples from other games or artwork).
- Try estimate the minimum and maximum polygons of models need.
- Texture sizes and extra textures like normal map or specular map.
- File formats.
- Animated or static, because this change the workflow completely.
- Additional level of detail models (LOD).
This topics help the freelancer understand better what you need and you can more easy compare the prices between the freelancers.
I hope this help.