Growing Mint in Florida

I have a produce broker who is telling me spearmint is pretty easy to grow and will bring $2/pound. He seems to believe you can pull 60-70 pounds out of a 4' x 100' ground box every 3 weeks.
I have a hard time believing this. He says he will buy everything I can grow. Is this a rabbit chase?

It's not impossible to achieve those yields, especially in intensive growing conditions such as hydroponics. But in less intensive field conditions the yields are less. Here's what we have in our ProGrowers database for field conditions:
"fresh herb 10-25t/ha, dried herb 1-3t/ha (years 2-3)"
These numbers are metric. To convert to imperial measurements, fresh mint yields range from 0.22 to 0.55 lbs per square foot, or 88 to 220 lbs in a 400 square foot bed. These are annual yields of fresh herb in the second or third year of the crop. In intensive environments such as a well managed raised bed with a longer growing season you should be able to double these numbers – at least.