jment, that depends on what business model you choose.
How much anyone earns is not a good question to ask in public as Dean points out.
You can rely purely on SEO (search engine optimization), spend no money on advertising and do well. You have a lot to learn, and you need to keep up with constant changes. It’s a lot of work.
Or you can generate income with advertising – usually you will see the higher the income, the higher the % of investment is on a continuing basis. This, too, is a lot of work since you need to watch your listings constantly and tweak all the time.
You can try to brand a site, but that takes a long time and a lot of exposure and capital also. This is the most expensive and long term model and becomes less feasible as time goes.
Of course you can also do a little of this and a little of that. Some people mix the methods within sites, I keep differently managed and designed sites for the different business models.