I paid ~$350 for a used S4 that I got activated on PagePlus. Why would I pay $80 a month to a big named carrier who keeps charging me rent on the phone after its paid off? (My plan is $39 for unlimited voice/text+1GB data, I used to do 500mb+1.2k min+3k texts for $29/mo).
Page Plus is truly amazing, especially now that they support 4G (but 3.5 Mbps throttle) as of a couple weeks ago. I just set someone up with a gently used S3 for ~$140, which if you compare specs to the S4, isn't much of a step down.