That's a hard one to answer really due to various conditions like the value of DEC or the cost of the cards themselves. I just read a great report yesterday that tries to explain it Here

What i will say is that its definitely worth playing and building up as many cards as possible before tbe Chaos Legion packs run out in a few months time At this point the value of the cards should see a substantial increase.

When untamed ran out last year i made loads of cash as everything went x10. I am not expecting such a big increase with CL as there are 10 times more cards in circulation but there are also more then 10x the players! I am hoping for at least a 5 times increase in value of all the good cards.

Buying up cheap Legendary and epic cards is a great investment.

I am currently playing in gold and i make roughly $20 - $40 per season but i am spending about $20 on rentals.

Peace Graham.