The interest rate varies based on collateral: https://oasis.app/borrow/markets. Eth is between 3-9% depending on liquidation ratios, and wBTC is at around 4.5% and both of these numbers are drops in the bucket when you consider that the collateral has appreciated a lot more than that.
And one more thing, it's not technically an interest rate, it's a stability fee (which btw, gets burned, used to go back to stakers, but they removed that).
Okay, so I suppose the stability fee is paid in MKR, and Dai's stability ultimately is regulated by people who buy MKR, and because MKR is burned by the CDP, people who buy MKR receive the "interest" indirectly due to deflation of MKR? Though MKR is independently traded and seems to move far more from trading than deflation.
And one more thing, it's not technically an interest rate, it's a stability fee (which btw, gets burned, used to go back to stakers, but they removed that).