I'm not really into grinding the market and flipping cards, I'd rather just play H2H. Which mode offers the best coins for a very mid level player?
If you primarily play H2H, just play seasons. You get decent rewards and if you're able to win a national title, you get the coins and natty packs which you can convert to coins by either selling everything or quick selling and rolling training rolls.
From what I've seen, if you're willing to play H2H, that's the best option to make coins. There are guys who are really good at the game who just play regular seasons instead of Gauntlet or Ranked because they can make a ton of coins.
Right now, I'm doing H2H seasons, and some Ranked.
Now that there is a weekly objective for Ranked that gives out two Field Goal packs and two Victory Formation Packs, I make sure that I win 10 ranked games a week.
And, if you do that Ranked objective each week, you should be able to get to the Champion Level.