Russian technology still has advantages. The Su-57 platform, if TOT is conducted, gives China insight into the LEVCON aerodynamic design. Likewise, there's a mature DIRCM technology on the Su-57 which China is only developing.I really don't know why you think China still desperately needs Russian TOT at all, not to mention the ridiculous cost analysis.
This generation or the next generation of aircraft is likely the last time China trails Russia in any technological capacity.
For the cost analysis, the F-35s are going to cost 1 trillion over their lifespan, but the airframe costs are only around 200 million, or in other words, the F-35's lifespan costs will be five times the airframe costs. If the J-20 is in the 100 million range (at current exchange rates), 600 J-20s could cost 60 billion base, but only 300 billion for sustainment.
As for the Su-57, I still think it's a vital purchasing option if the Russians can sell it cheap, given the decline of the RUR compared to the RMB. I don't see it as a particularly capable system, but it's an excellent skeet aircraft in the same vein as the F-35. However, an Su-57 isn't going to obsolete a JH-XX or J-35 simply because of its stealth penalties; for the PLAAF to have options between a heavyweight and long-ranged Su-57 and a lightweight but stealthier J-35 allows it to diversify its force structure and make it more challenging for opponents to counter it.