There are two types of currency within FIFA Ultimate Team--Coins and FIFA points. They are shown on the upper left of most Ultimate Team menus.
Coins are earned through playing games or selling cards via the Transfer Market. These coins can be spent on packs or cards on the Transfer Market. In other words, you want coins. Lots and lots of coins. Good, sweet, nectar of Ultimate Team life.
FIFA Points are purchased through the PS Store or Microsoft Marketplace with real money. These can only be used to purchase packs from the in-game store. Also, while these points can be used to purchase packs via the Web App and FIFA 15 Companion, they cannot be purchased there. They can only be purchased on the console. You don't want to use real money unless you're rich or... well, that's it. Unless you're rich.
FIFA Points can be bought in multiple denominations. 100 FIFA points will cost $0.99; this is enough to buy two Silver Packs or one Gold Pack. Point purchases can be made from $0.99 for 100 points all the way up to $99.99 for 12,000 points. Yes, that's right. With 1 click of a button you can drop 100 bucks for digital trading cards. Don't do it!
You can have a lot of fun playing Ultimate Team without spending one dollar. It might seem to be a smart investment to purchase the Silver and Gold Upgrade packs when you start playing Ultimate Team, these are the packs intended to get you into the mindset of spending money. As you can see they are priced in a way that makes them not work purchasing with Coins. 15,000 coins for a Silver Upgrade pack would be much more wisely spent on six regular Silver Packs. If you’re willing to spend a few dollars, the value in those packs is there, because the Point Price of 50 is equal to the point price of a regular Silver Pack. Don't do it!