What can be won in the arena?
You get up to 5 rewards on completing 12 wins or 3 losses. One reward will be a card pack. The other one to four rewards are random (dust, gold, single cards). More wins = more and better random rewards.
What is the minimum price received when loosing 3 times?
You always get your card pack. The worst random reward is "5 dusts". However, there is MMR matching in arena. You should expect to win at least one game on average even if you're terrible at picking cards.
When accumulating gold through daily's what is the best way spend this gold?
- Dailies in play mode generate 1 pack per ~3 games.
- Arenas generate 1 pack per 5-15 games played.
- Games in Play mode without dailies generate 1 pack per 30 wins (now limited to 1 per day).
There's two extremes of player here: Some players complete the dailies and log off, sometimes going for multiple dailies at once (3 or fewer games per day). On the other extreme, others play at least two hours or more each day (8 or more games per day).
- If you value your time highly (as the first group does), doing the dailies in play mode generates more gold than arenas. Arena purchases wouldn't get used, so packs should be purchased instead. You may want to set aside 300 gold in case you get an extra burst of playtime so you can arena on those days.
- If you value your time playing at a net of zero (as the second group does), doing arenas generates more packs than play mode.