I'd like to know what is a reasonable price to pay a garage to have this sorted out? Also, it's been like this for several months, might it progressively get worse? What's worse case scenario?
My car has 25000 miles.
My car has a manual transmission.
This sounds like a door lock actuator is failing. When they begin to do this, they can continue to work for months or years, but will eventually stop working all together. Keep in mind, I am not there to hear the noise, so rely on one our mechanics to make a final diagnosis.
The description of your car only states its a Nissan. We will need more information to give you an accurate estimate. I’d recommend having the vehicle looked at by a qualified technician who will be able to diagnose and replace your door lock actuator as necessary.