Hoo boy if I were quoted $150k for 5 ac I'd sure want to know why. 160 ac drainage is massive and dealing with that can be a challenge so maybe that's what he was anticipating.

If you have too much watershed you can be in a tough spot. Even if you have adequately sized overflow and emergency overflow systems that can withstand the deluge, your water chemistry can go wildly out of balance every time you get a significant rain event. You're likely to replace the entire volume of the pond several times over each time it pours with adverse effects on your fish population.