If it was cheaper to get cows from Africa than to breed them then you can bet your ass people would do that.
Economies of scale and/or new techniques make farming donkeys cheaper.
Restrictions are put on the import of donkeys from overseas.
Restrictions are put on the export of donkeys to China.
The amount of donkeys in the world get so low that the price naturally rises (yikes).
The demand for donkeys is reduced somehow (unlikely)..