K, my apologies for the somewhat belated response.
Assuming a typical service life of 30 yrs. for the average nuclear sub, I would assume refuelling and modernization costs of about $200 million (in constant dollars) at the half-life stage. If towards the end of its service life, another extensive refit and refuelling is done to the tune of about $400 million, that would extend the life of the average nuclear sub by about 12 years. That, over and above the average annual maintenance costs usually equating to between $20-22 million, would imply a total life-service cost of approximately $1.5 billion. For a USS Seawolf (SSN-21) class nuclear submarine, with acquisition costs of about $2.1 billion, that totals to a lifecycle cost of about $3.6 billion (again, in constant dollars). Or for a Los Angeles (SSN-688) or USS Virginia (SSN-774) class nuclear sub, with average procurement costs of about $1 billion and $1.6 billion respectively, that would total to lifecycle costs of about $2.5 billion and $3.1 billion respectively.
On the other hand, typical per unit procurement costs for submarines using air-independent propulsion systems such as HDW [Howaltswerke-Deutsche Werft] or Thyssen Nordseewerke [TNSW] systems vary anywhere between $100 [the Swedish Gotland class diesel-electric featuring the Sterling AIP] to 500 million dollars [India acquired her 6 Scorpenes for USD 3billion, albeit with full ToT]. Mid-life upgrades and servicing costs also vary significantly, but based on the British 'Upholder' (Candian 'Victoria' / Type 2400) class submarine, retrofitted with an AIP system, annual per-unit operating costs (based on its 'Oberon' program and a highly conservative estimate that factored smaller crews and a new Upholder technology) were estimated at about $22.5 million each, and routine overhaul costs at $250 million over the life of the submarine. Bear in mind however that the submarines were purchase second-hand and at a cost of $152.5 million each, but if their mid-life refit and servicing costs are extrapolated [debatable since AIP technology has made rapid strides in recent years] to say the Scorpene class submarine, that would represent a total lifecycle cost of between 1.4 - 1.5 billion in constant cost dollars.
In addition, the HDW and MESMA systems are extremely quiet- far quieter than any nuclear/steam plant infact, rendering an added stealth advantage to AIP submarines in minimizing acoustic detection by underwater passive sonar arrays.