Take Buxus (Box Hedge) for example. Here in New Zealand, they cost about $2 a plant (10 inches max). If I spend $50 on these, I have 25 10" plants. I plant them and allow them some growing time. I could even take some cuttings off the small plants already.
So 25 plants one year later will be roughly 20 inches high. I take 5-10 cuttings from each plant. Now I have between 125 and 250 cuttings. Plant these, expect 80% survival, and I now have 100-200 plants. I can sell my old plants for $4-$10. 25 plants at $5 is $125. Unless I want to keep them and grow them some more, take more cuttings, repeat. Throw a bit more time at it and I can sell my 100-200 cuttings for $5 each. Now I have $500, a bunch of plants and close to nothing in cost. You can sell them online, contact a nursery, go to gardening shows. Nearly anywhere and people will be interested.
But remember these all started from the little 10 inch $2 plants. Doesn't seem to hard does it? We all have to start somewhere. Making money takes time. Plant 500 cuttings this year and next yeah you could make an extra $2000. The following year, $10,000. So on and so forth. Enjoy, leave a comment or question!