Yes, it's a fantasy. You actually have no idea the specifics of the comparison, for example your "60-80%" which you obviously made up out of thin air. We are also NOT going to see China's economy grow to double the size of the US in 10-15 years time, another fantasy that you have no support for, and especially with the exchange rate now being increased as a trade war weapon. And please don't try to pull a fast one by using PPP instead of nominal GDP in this fantasy. Try buying any internationally-traded commodities (such as are used in ship construction) in 'PPP dollars' and see what happens. You'll get some savings from 'economies of scale' and labor, but you really have absolutely no idea how much this is, much less derive a future PLAN fleet strength from it, and whatever benefits can be gotten from these advantages certainly do NOT translate into China's full GPD, either nominal or PPP.
The US military has a rule of thumb that a doubling of the procurement rate typically results in a 20% decrease in cost.
Depending on how you interpret this, this translates to cost saving of 20-40%
And for a technical service like the Navy or Air Force, equipment procurement/maintenance costs account for the majority of the Total Lifecycle Cost.
There are published studies on this. For lack of any better analyses, this estimate will suffice.
PPP is the correct way to measure military spending for China.
The vast majority of Chinese military development, production and operating costs are incurred with a domestic Chinese cost base.
We can see this with the published export prices of various Chinese military equipment.
---
Your commercial ship construction comparison as a internationally traded good fails.
Look at the cost difference between Chinese yards and US yards for commercial ships.
And the US military is not going to be producing ships overseas, but will be using high-cost shipyards located in the USA.
---
Estimating future PLAN fleet strength is a big question, but from a resourcing point of view, as I said previously:
1. My low estimate is now 3 destroyers per year - which results in around 100 in total.
That just aims for parity with the US Navy.
That would already be "affordable" given China's GDP is 30% larger in terms of PPP.
2. My high estimate is 6 destroyers per year - which results in 200 in total.
That aims for twice the size of the US Navy, which would be more than enough for overmatch.
With economies of scale, this would cost 60%-80% over the low estimate.
And would be really affordable if China's GDP grows to twice the size in 10-15years. And that only needs China is sustain 5-6% growth per year.
Sure, you can disagree with the high estimate of 200 destroyers, but let's say China only aims for a fleet only 30% larger.
That is almost certainly sustainable given economies of scale and how China's GDP in already 30% larger today in terms of PPP.
No matter how you look at it, the point remains that China should be able to build a larger naval fleet that could dominate the 1st Island Chain and then beyond.
And from a requirements perspective, China will probably build a larger navy because it wants:
1. Military security for the mainland Chinese coast
2. To protect its global trade/investment interests (it is the largest trading nation)
3. For Taiwan contingencies