Assuming you can find the parts it shouldn't be an expensive job, but understand that "expensive" is a relative term. It will be expensive relative to changing the air filter, but it won't be expensive relative to changing the head gasket. If you are pretty handy with tools, you could probably even do the job yourself and save some money.
I imagine your Micra is a 4-cylinder with the valve cover on top, easy to get to. From a parts standpoint, the plug wires will probably be the most expensive part. Here in the US if this is like a typical 1994 Nissan 4-cylinder, a person could probably get all the parts for the valve cover set, plug wires, air filter, fuel filter, throttle body and MAF sensor cleaners, and a bottle of fuel injector cleaner for under $200 to do the labor themselves.
-Rod