I've actually played this game last year, and I did had quite a hard time earning money since I never really spent anything with real money back then. But now, it's extremely easy.
Spam myth and sell items, it's not that hard. If nobody buys the stuff you sell in the market, just NPC it. Myth isn't hard with a good team. Earn enough money to buy better myth equips like Seres or Soldin and you're good to go for soloing m1-m3. Rebirths are really only for the skill points; once you have the correct skills for myth for your character, you can pretty much do good. e_e
I hope this doesn't come off as bragging but I started a week and two days ago and I my Yuki is already 82 at 2nd rebirth with Soldin equips lol. Before, my Tia couldn't even get past 75 because myth was too hard and I couldn't earn enough money, but now it's much easier. I've only spent 10$ on the game so far and I've spent them all on fashion stuff lol.