I've been very happy with MyFamilyMobile from Walmart. They use Tmobile network. $30 for first line unlimited talk and text. $25 for additional lines. $10 to add unlimited data per line - includes 5 GB of high speed data, then it drops to slow speed data. You need to buy their sim card at $25 a piece.
I have three phone, and only one of them has data added on. Comes out to around $110/mo with taxes and fees included.
As a side note, I am the one with the data added on. I don't need 5GB of data per month. I use on average 2GB, floating between 1.5GB and 2.5GB. However, I looked at other plans and they were offering either only 1GB or only 2GB of data (or high speed data) for the same price. My typical data use is email, browsing and reading message boards and forums like this one, but I keep track of about 4-5 of them, facebook, google maps for locations/directions, weather, instagram, playing mp3 files from dropbox (need to copy them onto the phone, but keep forgetting), podcasts, etc. I use more data if I want to stream netflix or youtube. Those can eat up data really fast.
Basically, I find this plan to be comparable in price to most other low cost cell service providers, but gives me a bit more flexibility for when I do need (or want) to use more data.
I tried Ting on my kids and didn't like it. The whole pay per bucket structure doesn't work for me. I tend to stress every time the threshold for each bucket comes close. It's like, if you go over by just one txt or one minute, you have to pay more, but then if you are in the next bucket, but only have few txts or minutes in there, you grossly overpaid for those txts/minutes. And with teens, I can't, and don't want to, control their txting amount. I have much more important things to deal with and control when it comes to kids.