It is good for China that Russia is willing to sell AL-31s ..... I don't think this deal has anything to do with trade deficit.
123 x (2 or 3 million USD) = 270 MM USD (approx.)
And that over 3 years .. is only 90 million USD per year.
In fact, even if available, the WS-10A etc. would be atleast as costly to make at AL-31 .....
Most likely, (even if available) WS-10A would be quite costly to make right now .. given really low volumes ..... and the quite large volume production of AL-31.
This part of the relationship is more in favor of China, than in favor of Russia.
Further, at present, Russians given a damn to how many J-10, J-11, J-12 .... J-99 are flying near their Asian borders.
They really want a cool money making opportunities .... anywhere in the world.
Russia would be more interested in selling the complete fighers, which could be a 10 or 12 billion dollar order.
(Agreed, China is unlikely to oblige them with a reasonable money making offer, anytime soon .... but they can dream
).
Engines are just to continue the relationship etc. ....
they don't make any money out of it.
The Russians and Chinese both understand this basic fact.
And, therefore China needs to keep the Russian morale (and high hopes) intact .... before they get bored with this peanuts kind of stuff.
(in money terms).