How much does an iPhone 4 cost?

Being the stupid person that I'm, I accidentally dropped my iPhone 5 into the toilet and is currently drying in a sandwich bag full of uncooked rice and silica gel packets. I do not have high hopes, so I'm in the market for a new phone. My iPhone 5 was bought for me as a gift, and I do not want to pay for another one. So, this led me to want to buy and iPhone 4. How much would it cost? Also, should I get a regular iPhone 4, or an iPhone 4s? Is there a difference?

Why can't my iPhone text one specific person?

This started happening about a week ago randomly, my friend who has a Note 3 stopped receiving my messages. I get all his messages, but he doesn't get any of mine. I've tried turning off my iMessage but that didn't work. It's really frustrating because he is the only person that I can't text. I have been texting other people (iPhone and non iPhone) and it's all working the same except when I try to text him. He s tried to reset his phone aswell but that didn't work, and neither of us know how to fix it.