Aeroflot calculates prices by route, fare type and bucket.
I’ve just done a stop over on a trip from HKG via SVO to AGP in flex fare bucket A. No extra cost at all were applicable.
If your fare type (e.g. promo) only allows a stopover for a fee, or if the bucket isn’t available on your new flight date, then the price will be higher. Same for most airlines I fly with (CX, IB, MU).