You may be confusing "rustle" with "buy"
Rustle - "To steal livestock, especially cattle"
It may well be that my English isn't perfect.
What I mean is that even thogh I like the idea of herdsmen and think its a great and original idea it could also easily be exploited (especially since AI players don't know how to use them). An easy (?) way of nerfing the herdsmen would be to make make the "rustle" action cost an amount of gold.
\Skodkim