The Type 99's 2.6 million USD quote was from 2005. Factor in the inflation from 2005 to 2019, that would be 3.4 millions. Consider that the VT-4 export mode to Thailand is 4.8 millions with downgraded armor and no laser soft-kill system, although the VT-4 does have RWS, automatic transmission, newer FCS and LWS. The ZTZ-99's price is mostly an internal price with no profit margin, so if you use the VT-4's price as the factor (assuming it has the 2nd gen TI, not 3rd gen as in the 99A), the actual price for the ZTZ-99 (the late model with 2nd gen TI and digital battlefield management) should be around 4.6 millions in today's price.
Now factor in the cost of living and purchasing power index, which China is 1/4 of that of the USA, so x4 of 4.6 millions, the true cost of ZTZ-99 is about equivalent 18.4 million to their economy. What I mean by internal price is that it's the material cost to build the tank, but the salary and pension of the workers, the energy bill, the facility cost, the R&D costs are all subsidized by the government in one form or another. Also consider that some material like ballistic steel and germanium lens are priced about the same doesn't matter where you at. So that $1500 germanium lens for your gunner's sight is now costing a equivalent to $6000 for the Chinese to acquire, although they may have a small discount by locally produce it.