Mine, A lot, around level 20 miner you earn some serious money!
Don't think rich people make 100k in 15m. They grind or used to grind for 4 hours a day to make the money they have!
its true, having a few houses myself (not saying I'm rich) its not like a secret thing, i fish and mine and sometimes farm. It takes effort but once you get some money, multiplying it by renovating houses, stock market, gambling ect... is easy.
Do the miner job with efficiency 5 and fortune 3 on a smelter's pick. You'll get like 2-5 ingots per iron or gold ore and TONS of other stuff. Make sure to alternate between the pick and shears, and sell the profits at /warp mall (Make sure to craft them into blocks if you sell at the mall)