To talk about your sentences, 1. correct. But it sounds odd because it's saying two closely connected things with "on the other hand". 2. The punctuation is not correct. "and" is never isolated with two commas as you have done. You could rewrite it like this. 1. ..., the price of things will increase. And when the cost of production decreases, the price of things will decrease. 2. ..., the price of things will increase, and when the cost of production decreases, the price of things will decrease. 3. ..., the price of things will increase, whereas when the cost of production decreases, the price of things will decrease. Generally, "and" connects two clauses with little nuance or effect. "while' and "whereas" have a greater contrasting effect and sounds better in this kind of sentences. All of the above don't sound particularly good though. It might be better to restructure it completely. - Increases in production costs lead to general price increases, while decreases in them lead to price declines.

Hi Fhon, The phrase "On the other hand" is used to highlight or contrast two different things, usually an opinion. For example, "I have $100,000, I could invest a large portion of this now in the hope of a greater return, on the other hand, I could keep on saving until I can make a greater investment." "We could go for coffee somewhere in the city, on the other hand, we could go for a coffee near your place, I think it will be much quieter & nicer there." "I'm was thinking of going to Granada in Spain for the weekend in Semana Santa because I think it could be a lot of fun & very relaxing, but on the other hand, it could also be very crowded because it is Semana Santa & there will be a lot of people everywhere making it less relaxing." Hope this helps, maybe you can try again with me with a few other examples? Nick