Here's what your friend means, for 150 gold you get an arena run which even if you don't win a single game gives you 1 card pack which you would have paid 100 for, now if you do well at about 3 wins you're guaranteed at least over the value of 150 gold so you've already made back your investment. At 7 wins you get enough gold to buy another Arena run outright on top of the pack you get and on top of any dust it decides to give you. So if you're decent at Arena you can keep going for quite awhile while accruing a ton of card packs, gold and dust (and sometimes at the higher win marks another pack and sometimes gold cards).
I personally made most of gains from Tavern Brawls, Arena, and lastly a bit from Ranked play but that's definitely the least profitable.
Oh and Andy if you were going to craft any card pre an expansion Tirion is probably the best, I certainly don't regret crafting my Tirion.