I'm not talking about absolute cost, I'm talking about opportunity cost. IE: The same ICBM can either launch 6 warheads on conventional trajectory or it can launch 3 warheads on FOBS trajectory. Which is better?
The answer depends on how good ABM is. I think it is wise that they experiment it now, because US ABM may be a joke right now, but they may git gud in the near future, FOBS + HGV can guarantee China's deterrence is future proof.
I don't think FOBS really change the MAD picture that much, US and China both has IR based Early warning Satellite, those can easily detect a launch FOBS or not. Even if they failed to detect the launch, there is still sub based ICBM for 2nd strike.
at the very least, a credible Chinese FOBS would be a potent bargaining chip for China during any negotiation with the US. The Soviets leveraged the R36’s FOBS capability during arms control negotiations by offering f to trade it away in returning for the US IR detection and processing technologies which enhanced the effectiveness of Soviet early warning detection. The logic being if Soviet Union were to allow the US to become more secure from a Soviet first strike by forsaking FOBS, then the US should also help the Soviet Union become more secure from a US first strike by giving it technology to improve its early warning.