Pay comes down to 4 main elements:
1. Collections per patient (a very important number that you have little control over as it based mainly on demographics)
2. Patients seen per hour
3. Operating expenses
4. How much management (if there is one) is skimming off of you.
Example 1: If your level one trauma center collects $80/pt and you see 3 pph then you are generating $240/hr. If you're working for a contract management group that pays you $175/hr then they are keeping the other $65/hr to pay operating expenses and increase their profit margin.
Example 2: If your level two community ED collects $100/pt and you see 3 pph then you are generating $300/hr. If you're working in a true small democratic group that lacks any sort of oligarchy, then they would pay out all expenses (malpractice, banking, billing, health insurance, mid level salary, etc) and distribute what's left over to the docs probably yielding ~$250/hr.
Simplified but hope that helps.