What's there to think about? It's all up to you, how much you are willing to pay, how much they are willing to accept, and how good both your bargaining skills are.
Some foods for thought .... :think:
Who should command a higher rate (not necessarily as what $ you are willing to pay)?
1. An experienced (as in able to pose and natural) part-timer model, vs a full-time, accredited (as in with a famous agency) average model?
2. An average looking, accredited model vs a VERY good looking newbie model?
3. A model before vs after she wins a beauty/model contest (and the skill is still intact, but not the reputation)?
4. A model doing sexy(as in showing as much skin as in bikini or lingerie) fashion shoot, vs bikini shoot vs lingerie shoot?
5. A part-time model (say a student) vs another part-time model (say a SIA girl)?