If you are looking to battle for DEC its really worth looking at getting Gold foil cards vs regular ones.
Here's an example why. If you take the Chaos Knight as an example. In Bronze League you'll need 14 regular cards or 1 regular one to get the lvl 3 cap.
A regular card is currently priced at : 24 cents x 14 cards : $3.36
A gold version of the same card (lvl 3) : 1 x $4.24.
A difference of 88 cents. If you average 10 DEC / win that extra 10% is an extra DEC every time she fights. 88 fight and she's paid for herself!
In some cases the GOLD card is actually cheaper than the regular card at similar level then its really a no brainer!
Rare cards tend to have a higher dollar difference. Epics even higher difference and Legendary is a huge difference. Gold cards rarely are worth it there.
Summoners are a weird one. There is a huge difference in price but you will use them each and every battle. I tend to think they are worth spending the extra money on.
Final thought. If you are doing guild battles there are gold foil only battles. If you can field a good Gold team you won't have to fight with your guildmates over the less attractive fray and get more frays in :)
Happy hunting everyone