There's also another way to look at this besides the matter of cost of development, in my opinion. Right now they are using J-15 which basically Su-33 based. That's a large carrier based aircraft, that even Russia now seems going to change their Su-33 with Mig-29 derivative as their carrier based fighter.
J-20 will be more or less in same ball park dimensionally (even bit slightly larger), and both are bigger than FC-31. Perhaps shown tendency of PLAN on using large relatively longer range fighter from their carrier.
With STOBAR on Liaoning and Type 001A, the J-15 is limited on take off weights thus can not exploits it's true range.
With CATOBAR on their next gen CV, J-20 can exploit more of it's range, especially if China also developed carrier based Tankers.
It's like USN used F-14 as their sole carrier based fighter.