Taiwan's participation/admittance to UN signifies independence. Non-support of Taiwan's participation is useful leverage for other nations when they need to negotiate any agreements with China. It's the classic Taiwan card. Besides, it's not the first time PRC has paid for countries to maintain one-china policy. Some African nations even brazenly asked and received money for not recognizing Taiwan.
According to Jing can rong (金灿荣), influential Chinese IR expert, China pays 300 billion RMB every year for other countries' non recognition of Taiwan. This is from his official youtube channel
Unfortunately, the talk is in Mandarin. The relevant part starts at 7:45 where he talks about the economical cost of not getting Taiwan back.
For members that understand Mandarin, the entire video is pretty useful as it lists ten benefits of Taiwan's return across from many areas including ideology, scientific development, geopolitics, military and economy.