From the Russian's point of view, they need to sell the Su-35 as an entire package with Russian avionics and weapons. The Russians can't nickle and dime the Chinese with just an airframe. From Chinese's point of view, the Su-35 is not good to buy for those exact reasons. There will be no way for Su-35 to use Chinese data-link and weapons unless the Chinese completely gut the Su-35. However, if China did that, then China had to spend money on things that immediately go in the trash bin.
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From what I heard, Russia finally relented and willing to change to accommodate China's request.
Even at worst case nothing got changed Su35 would be a designated superiority fighter with only A2A missiles (forget bombs, AShM and others) which China can purchase from Russia. 24 planes aren't a whole lot.
2 Billion for 24 planes is not that much money. Japan just purchased 3 US predators drones for 1.2 Billion. Therefore China willing to get the SU35 to be used as reference.
China wants to pit J20, J31, J11D against SU35.
Finally it wants to get some clue about 117S engine.
Even Su35 don't go into actual combat, China still willing to pay for it as reference.