Black smoke from exhaust pipe and mileage became half

We do not have the car/model in US, but the thing to do here is to check fuel pressure with a gauge the regulator may be acting up or it may be a plugged return line going to tank if it is fuel injected.see our link however we have no specs so you will need check into what it is supposed to be.
https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

We have virtually no information on that vehicle as it was never exported to the US and the Suzuki it is based on was not shipped here either.
However, black smoke is excessive fuel into the engine. My guess is that either a fuel injector is stuck open or the pressure regulator in the fuel system has failed if you have the fuel injected engine, or if it is the carburetor version you have a float issue causing fuel to spill into the engine.

Cost depends greatly on what the issue really is. If it is a sticking or failed injector it should not be that bad.
You will also want an oil change as soon as it is repaired as the excess gas will also be getting into the crankcase and diluting the oil.