Because the Jiang Family did not lack money, what they lacked were imperial appointed officials who could bring honor and benefits to the family. Presumably, the Jiang Family would still be very willing to spend money to earn administrative achievements.
In ancient times, oxen greatly benefitted agriculture and farming and were the only national-level protected animals. Imagine, if a large number of oxen appeared in Qingyuan County during Jiang Yifan's tenure, then the farming in Qingyuan County could climb a small step. This step would be the increase in grain production, the increase in tax revenue, and also a hard index of Qingyuan County's outstanding achievements.
Likewise, His Highness Prince Yu also didn't care about the small cost of starting a cattle farm, but in the long term, if the cattle farm could be established in Yuyuan Prefecture, the biggest beneficiary would be Prince Yu, whose fief was in Yuyuan Prefecture.