You can get money by farming items that drop off monsters. Farming is just the collection of the items.

For instance, you could kill crawlers for their silk thread (i.e. farming silk thread), and sell the silk thread in stacks of 12 for a decent amount of gil.

Most servers have different prices for different things, and it's all relative. The economy, just like real life, goes up and down all the time; due to supply and demand. So I can tell you I can make 200,000 gil in a couple of hours at level 40 by farming tree cuttings in Passhow Marshlands, but another month of doing the same I could probably only get 100,000 gil in the same time period at the same level.

Usually when things go down, so do higher level items such as Hauby's and Scorpian Harness', so it's all relative in that respect.