y'all, he's gonna basically have a monopoly, or dominate the market as one of few suppliers for moongrass, enough to be able to be the one setting the price (oligopoly). If he has enough market power to be the one who can set the price and bring people to shop from them, then even if they produced a billion moongrass plants they'd still be able to charge a lot for it. COMPETITION drives price down, so no matter how much moon grass he plants, the value doesn't drop just because there are a lot of it. It's the amount on the market. If he's the sole or one of few sources of moongrass, doesn't matter how much he has; he controls how much is on the MARKET which means he can artificially make it super rare to obtain.
MC should definitely also make some farmland to be able to sustain their own food source, but making food be their primary use of land will not generate enough income for the get rich scheme he's going for. Those markets are saturated and will not have as much demand.
If moongrass generates SO MUCH income and has long term monopoly potential, then he can use the income from it to buy food for trade.
This is why people should take an intro econ class at some point in their lives :3 It makes us all better, more informed citizens living life in this world!