Although I hate to be the bearer of bad news, the short answer is probably not much. Elvis Presley memorabilia falls into a couple of categories--one, the readily available mass produced merchandise that was a dime a dozen when it was produced and is still easy to find today and two, the original, one-of-a-kind items that are few and far between.
Unfortunately for most of us, the collectibles that we typically unearth in an attic or at a garage sale fall into the first category. That limited edition Elvis ring, the numbered trading card, and the Jailhouse Rock statuette are all essentially worthless, at least from a monetary standpoint. Sure, you might be able to sell them on Ebay for a few bucks or at your next yard sale for even less, but that's about it.
That is not to say, however, that these collectibles are worthless in every way. Value is in the eye of the beholder (or the collector, as the case may be). Perhaps that statuette is part of your collection of Elvis figurines. Maybe you have fond memories of Grandma's Elvis cookie jar. Even if the memorabilia won't make you wealthy, it may still have special meaning for you which is certainly valuable in its own right.
All of that being said, there are authentic, one-of-a-kind Elvis items out there that are worth quite a bit to collectors. For example, an empty prescription pill bottle belonging to Elvis sold a few years ago for nearly $4,000. Likewise, original album art for one of Elvis' records sold for nearly $5,000. Then there is the jar of Elvis' hair (yuck!) that sold at auction for a whopping $16,000.
Unfortunately, most valuable collectibles came from friends and associates of Elvis Presley, meaning you are unlikely to randomly run across such items in an attic or at a yard sale. Nevertheless, anything is possible and you just never know for sure what you could find. Until you stumble across Elvis treasure, though, enjoy the collectibles you have for the joy they bring to you. And if they bring no joy, you can always sell them on Ebay for a couple of bucks.