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If it costs $30 to produce a barrel of oil in country A, and $70 in country B, would oil price dropping to $60/barrel affect both countries equally?
Of course not. Country B would have to sell its oil at a loss
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Right. Even if it cost country B $59 to product/mine a barrel of oil the effect of $60 oil would not be the same. Country A would see a much smaller decline in net oil revenue then country B
In your example country B would need to stop mining oil until it's price rebounds for a somewhat long term before it would make sense to resume mining which would result in lost jobs and a slower economy