If current consumption of serial flash SST26VF064B is 15mAq104MHZ then at 50MHZ it is 7.5?
The answer is in the datasheet



The read currents at different speeds are noted and they are clearly not an ordinary linear curve.

When the interface is idle (no reads or writes active) then the current consumption is 45μA max.

The total current consumed is (assuming no writes for now) Read current * duty cycle of reads (what percentage of the time are reads being performed) + standby current * 1 / duty cycle.

The write current does not change with interface speed (see the datasheet for details). 

If you are doing writes and reads, then add the average write current to the average read current and add the average standby current.

As those averages are use-specific, I cannot help any further.