This is what happens at many places, bonuses and airfare compensation are extra costs that many "schools" cannot afford. Hence, all the different pretences why not to pay them.

What you term 'extra costs' are specifically detailed in Chinese state-issued teaching contracts. Said contracts are supposedly signed in the spirit of friendly cooperation. That is indeed mentioned explicitly in them. The Chinese side looking for lame excuses not to pay those extra costs to FT's at the end of their contracts is not on, no matter whether they are affordable or not. If not affordable, the relevant clause concerning their payment should not have been inserted in the contract offered to the OP in the first place. What he has described is sharp practice, which invalidates the contract as that flies in the face of the spirit in which the contract was supposedly signed by both parties initially. If that kind of thing ever happened to me, I'd go off my trolley! Just my two cents' worth!