It is much cheaper than maintaining a conventional force needed to achieve the same deterrence.
Cost wise Britain spends about 3 billion a year, France 6 billion (best comparison since they are a completely in house nuclear power), and the US 75 billion.
Based on those numbers thats about 1/6th to 1/10th of the defence budget. Pretty reasonable to me considering we struggle to even effectively generate a division for the Army, a squadron for the Navy and a couple squadrons for the airforce off that.