I'm still in my parents family plan with AT&T. They lowered a lot of their service prices to attract people, but they now use AT&T Next, which makes you pay for your phone in monthly installments. I'm getting 15 bucks pulled out every month for my LG Optimus G Pro. By the time I'll be done paying it off my total will be around $350 in Nov. 2016, for a device that came out in Apr. 2013. Hell, what's left on my installments is what it would cost me to just go buy another one used. I did get it in Nov of last year and it was the cheapest smartphone they had that wasn't a piece of junk. I'm hoping I can make this thing last as long as possible given how much I'm spending on "outdated" hardware. It performs very well for everything I need, which is almost entirely reddit, Chrome, Snapchat, and texting. The camera is kinda eh, but I don't use it enough to be bothered by it. It seems to be holding up quite well though.
Mobile phones have seen explosive growth in the recent years. It's starting to fizzle a bit in terms of "bigger and better" though as of late. Phone can't get a whole lot bigger, and for what most people use them for the extra cores or RAM aren't really needed. My S3 was a dual core, my LG is a quad core, and I know they've got octa-cores with 1440p or higher displays. It gets to be a bit pointless unless you're playing games a lot. My friend's G3 is remarkably similar to my phone in a lot of ways, but mine actually seems to perform better and have less quirks, despite his being newer, more powerful, and a flagship vs an AT&T exclusive phone most people have never heard of. I have trouble finding cases for it even.
As Troncoso said, your phone sees a lot of abuse and any device that is used day in and day out is going to only get you so far. I'll run this thing until it has enough problems to motivate me to upgrade. Also I'm seeing removable batteries become more and more scarce, which is something I really prefer to have. Given that it has a removable battery, I could probably make it last a pretty long time. I had my S3 for 2 years, and by the time I had it for a year, it was already starting to get a bit quirky. It was a refurb through AT&T though and cost $50 as a one off, so I'm not too torn up about it. My LG was nearly 50 for the down payment and first month, not to mention all the months of paying for it afterwards.