From my understanding of Bernie's "Medicare For All" bill (S. 1804) [1] payment to providers would be fixed at the, well, Medicare rate. Medicare rates are about 40% lower than private insurance (about 56% of national healthcare spending is private), but it's also higher than Medicaid and obviously higher than what the uninsured pay. All-in-all, from the figures I've seen [2], this specific single-payer plan would lead to about a 10.6% [3] overall reduction in payment to providers (some of which will be ameliorated by a reduction in costs that maintain complicated insurance administration work at hospitals) but I imagine providers will generally take some percent hit to payments which would in turn affect physician salaries (albeit not drastically)

[1] https://www.congress.gov/bill/115th-congress/senate-bill/180... [2] https://www.mercatus.org/system/files/blahous-costs-medicare... [3] https://www.peoplespolicyproject.org/2018/08/15/provider-pay...