In real terms it will be even bigger reduction than 20%. Russia is exporting oil and gas and importing anything even remotely complex. They are not even able to make cheese themselves.
What is coming to them with $25 oil is a sharp drop in quality of every consumable. The food they will eat will be barely fit for animals and their market will be flooded with forged medicine that won’t do anything. All the while Russians not affected will cheer that Putin the Wise used the fake medicine to kill off the elderly and thus increased the money in Russian state coffers by reducing the pension payments. I have no sympathy for Russians because they brought it upon themselves.
Coming to lots of places where oil is the basis of the boom - happened to Texas in the 80s.