This is a bit of a public service announcement since in the past month I've seen so many cards on the market that are half upgraded and selling for less than they could be sold for otherwise.
Many cards take as many as five level one cards to create a lvl 2 card. So if you get 2 basic cards and combine them together, you just have a mucked up lvl 1 card that isn't any more powerful or useful but now you have one lvl 1 card instead of two lvl 1 cards.
That brings me to the combine all button. It seems fun but if you ever hit it, all you have accomplished is lowering the value of your entire deck. Forget it exists.
It seems like you're going to push this button and become unstoppable. What really happens though is you end up with a bunch of cards that are half upgraded, did't become more powerful and are have a lower value to the market if you ever want to sell them.
Upgrading cards has specific requirements (which you can find in the stats column) and is not always a great idea. If you are in the wrong league or have the wrong summoner, they may not even be useful to you.
Sometimes the upgrade you get is not worth burning the cards. In some cases like Furious Chicken, there is no difference at all between a lvl 1 and a lvl 2 and while level 3 is sweet, you can't take advantage of lvl 3 until you get to silver and have a higher level summoner.
Before you upgrade a card, be absolutely sure you know what you're doing. Maybe rent the card at that level first just to make sure you can use it and it makes a difference because upgraded cards are useful to fewer people and therefore lose value immediately.
If you can't make this value back in wins, don't do it.