level 20, unfortunately, is incredibly quick compared to 45-50 and even 40-45. Leves are fantastic for making it quick however you'll use a lot of them unlike with crafting. I would expect to use about 50-70 leves getting from 20-50, then again that was pre-2.1 so I don't know how much they've changed it.

Most money made by gathering classes come from crafts using the unspoiled node materials to make 1-2 star items, or making items that require a lot of mats or a lot of steps to get the right mats which having a gathering class makes it a lot cheaper. It is literally like owning a business (crafting) and doing all the work yourself, i.e you don't have to pay for workers, meaning more profits! $.$