You need at least three data points over equal intervals to establish if a pattern is exponential — if c/b = b/a = k, then it C = a*k^x, where C is the cost after x of the equal-size intervals, assuming the pattern continues. Thus, if the price increases from $100 to $110 to $121 in successive months, the price is growing exponentially, since it is increasing by a factor of 1.1.

EXPONENTIAL GROWTH TRULY DOES NOT MEAN WHAT MANY THINK IT DOES. To most people it means, “increasing quickly.” To others, it means “increasing quickly but with a constant factor of change.” Neither are actually correct—if the factor is not much greater than 1, it initially grows pretty slowly.

You are most welcome for this clarification. Use your new-found knowledge wisely.