I am at level 51 and I am not rich. I have had less than 28,000 gold septims the entire game. The time I had 28000 was hard work and that was just so I could buy a certain home. My character is a kleptomaniac who tends to hoard everything he crafts and steals. So, I don't really spend a lot of time selling anything. And I only got to the point where I could invest in shops a few levels ago.
Becoming rich may be easy if you focus on speech and do the thieves guild stuff really quickly, but it can be tough to make a living if you spend your gold on health potions and smithing goods regularly.
Actually, you can make a lot of money while dungeon crawling - selling everything that's dropped and you can't use.
My main character - level 43 - was at 200,000 gold about 4-5 levels ago, and that's when I stopped taking everything back, and now I only look for "cash" or gems.
I don't focus on speech - no perks/skill points, no investing in shops, etc. I sell stuff as I get it and for what the trader offers.
Also, on my level 33 character - who has well over 100,000 gold - I'm already dialing it back to gold and gems only - it's just way too easy to make money. But also time consuming.
Good luck.