Like many have said before, if you buy packs, buy them for the event items you get with them (i.e. sword tickets in this event) and only if you are serious about competing in the current event.
Regular packs (premium pack 300coins, premium box 1500coins, premium megabox 3000coins) have the annoying property of sometimes yielding 3* cards. I hate drawing 3* cards so I avoid them like the plague. (btw, pack is one card, box is 6 cards and megabox is 14 cards)
Always keep in mind that the odds for drawing a 6* from a pack is 0.2%, and the 5* chance is 2%. If you are going to spend $30, buy 30TE in the shop and trade someone for a cheap 6*. You may need a little more than 30TE but not much, depending on the card. (don't buy Alexander, he sucks)
Sometimes they run something called a dream chest which only yields 4* cards and above. If you want to buy packs just for fun (like the lottery) then wait for a dream chest, you'll be somewhat less disappointed and the 4*s you draw can be levelled to 40 and sold for 1TE each.