The point is not about buying alone but maintaining it. The US uses its weapons with its alliance in NATO by invading around the world. China on the other hand has neither an alliance nor a war economy; it has a manufacturing economy similar to Korea, Taiwan and Japan which relies on production and sale of commercial products rather than just military hardware.
There's a stark contrast between the two countries.
You cannot apply the big economy-big air force logic just blindly.
Oh and our PAKFA plans have been slashed to 144 units.
144 * 4 still makes 600 5th generation fighters - Chinese economy 4 times as large as India
Also since China will be producing all this itself rather than having buy from abroad, let us add another 200 fighters which makes 800 - cheaper to make it yourself and not paying a profit margin
to your supplier.