Hi Maha, First of all, both cases are pretty much interchangeable in most cases. 값 and 가격 are almost 100% same; meaning "price". Ex. 값이 얼마에요? = 가격이 얼마에요? = How much is it? However, 고민 and 걱정 are slightly different. 고민하다 = to think hard in order to solve a problem. 걱정하다 = to be worried (just worried not considering any solution). Ex. 무슨 고민있어? = 무슨 걱정있어? = Do you have any concern? 그걸 어떻게 해결할지 고민중이야 = I'm thinking how to solve it. 그걸 어떻게 해결할지 걱정이야 = I'm worried over how to solve it. 뭘 살지 고민중이야 = I'm thinking/deciding which one to buy. Hope it helps.