I went through the same process not too long ago. I was asked the same question about about expected salary. This question is asked with majority of medical group, multi specialty, or ortho interviews from my experience.
Like others mentioned know the MGMA and AGME data on salaries for surgical foot and ankle pods at the 25th to 50th percentile. That is your base salary (200 to 275k). They are hiring a DPM because it's cheaper than a F&A ortho so do not get crazy. Focus on the "bonus" aspect of your salary. Ortho groups like production so you can take a base lower in the range and make up the difference on your collections. Each group has a different calculation of bonuses. My additional income is a percentage of the profit after I cover my expenses/overhead (Salaries of myself and anyone working for me, DME, supplies, building costs, etc). It doesn't have to be flat percentage. At the end of the day the group wants you to be busy and happy.
Do not forget to clarify other aspects of of the job like partnership, patient type, clinic times, CME, vacation, ancillaries, moving expenses, signing bonus, other benefits, etc. You have more negotiating power in the private realm. Don't assume or leave anything a mystery. Just ask for what you want if it is reasonable. I haven't been told no yet.
Talk to colleagues in similar practices or use this website for advice/feedback on different pay structures/contracts. Younger DPMs are very helpful. It only makes the profession stronger. Also forget the standard podiatry model of base and 30% after 3x base or whatever.