Why won't my iPhone 4 Charge? - 4

I have a white iPhone 4. I don't have a warranty on it either. So today i went to charge my iphone after the battery was completely drained, and at 75 percent i unplugged it to play with it. They i decided to charge it again, but it wouldn't charge. I thought the cable might be broken, so i used a new cable and it still wouldnt charge. Then I used that same new cable on one of my other apple products, and it charged perfectly. I tried plugging the iPhone into my computer but it still wouldnt recognize the USB. I blew into my phone to get rid of the dust, and i even looked inside the dock to see if there was dust, but i saw nothing. I don't want to reset my phone and delete everything though. I have apps and pictures i would like to keep and are not synchronized on my computer yet for back up. It was working fine, but when i went back to charge it, it stopped. I don't know what to do. How much does it cost to change a dock protector. And is there any way i can fix it without having to change it. Is there any other solution to this problem?

Something is either in the charger port still, and preventing the proper pins from connecting, or something is wrong with the battery itself. I would take it into an Apple store.