I bought a used SamsungS6 Active off of Letgo and app on the smart phone. I use Straight talk I have unlimited everything and I thing I get 5Gigs of 4G data before it lowers. I had a Sony Z2 before and the reception is far better with the same service with my Samsung. It runs off of AT&T towers. Costs me 45 bucks a month, My girlfriend has and Iphone through At&T and I get better reception than she does in many places. A lot of places have WiFi so you are not using you data that often and if you have internet at home with WiFi you are go there.
Most of the Apps seem to be free although some have adds in them I guess to help pay for the free part, some don't, I have used it with my GMG grill and it works well.
AS said craigslist, Amazon, if you are on Facebook they have local yardsale or classified pages, and I have bought off of Swappa.com
You can get a decent phone in the Iphone 6 or 7 or the equal samsung for 150-250 or thta is the prices I see locally.