Product prices vary because of a lot of reasons. When you consider each country has it's own strengths and weaknesses, trading internationally can have significant effects. 

I'll give you an example at a very high level..

Take for example, oil.  Supply is up whilst demand is constant, meaning prices have dropped, oil companies have less money, then invest less in their business and make people redundant, which then means people aren't spending as much money and so other businesses suffer. 

The government gets less income from taxing oil and petrol as the gas stations and so has less cash to pay for centralised services, less in fixing roads, social care and so on. 

All because someone decided to up their output of oil.