SU-35 started way before PAK-FA. It is a continuity/upgrade of SU-27.
PAK-FA is totally a new project.
Russia will never go back to invest in a new prototype which will be generation less than Pak-FA now. Same is the case with USA. This how things work. Russia kick started 5th generation project 10 years late than Americans even when they were mass producing and exporting 4+ generation jets.
What has Chinese done so far ? Copied Russian technology with non of its jet seen a combat or scrutiny of sales/exports to any foreign vendor.
This deal has proved everyone right who was sceptical of Chinese plunge for 5th generation jets and predicting crash landing, especially when they haven't Ironed out obstacles of 4th generation technologies.
In the hindsight of how Russia refused to sell inadequate numbers Chinese posters like
@tony4562; still have the guts to address SU-35 as so called wonder jet.
His excuse further makes it more ironic that J20 still in a developing stage and may not be ready till 2020, but rich China can afford buy 48 SU-35 to learn some thing new. Why would you buy a generation less platform which will be delivered to you in two years or more when you are already down 2-4 year with 5th generation prototypes already flying.
The cost of the deal explains how skimmed the whole package is going to be and why they are doing it.