Vitamin c is really one unstable ingredient , the minute you open the serum ,the vitmain c meet with air / water and then it is easy to get bad . so you don't know weather it keep the best appearance as it marked .
So there are a lot of different price of vitamin c product ,as there are a lot of different kinds vitamin c . with their Differences in structure and produce technology , the price of these ingredient is greatly different .
For example , 3-o-Ethyl Ascorbic Acid . it is more stable and long time effective and high absorb by the face skin