On Verizon they have the Edge program that allows early upgrade for the discounted price, and a monthly payment plan on phones but other problems as they do not allow a share data or trade. A simpler way to describe the different plans is "they are complicated and different" so it's best to run all plans with your personal requirements and then choose the lowest cost one.
Presently, I have 3 devices, 2 phones and one ipad on Verizon shared data at 4GB per month. The cost is $176 per month and I get a 2 year upgrade for the top of the line smartphones for $199 for the 6 and $299 for the 6+ plus a $40 upgrade fee. I think I'll be paying $549 for the 6+ with 128GB. I will probably keep any phone like the 6+ for 3+ years. I'm not one to upgrade like our friend Scott.
heck I wouldn't have jumped to the Samsung last year but my htc that was 3 years old broke! It was an emergency at a time before the 6 was out. I usually pay cash for the phones with the upgrade discount. Typically, I keep the old phones as after 2 years they aren't worth the effort to sell used. With the new discount I got last week, the monthly will be $166/mo.
We ran the Edge alternative and this just had too many problems and would not have saved me any money. The Verizon store guy verified my conclusion. If I go a period where I'm not traveling for several months I roll my data plan back to the 2GB and save $20 a month.
I have checked the prices offered typically on a used but newer smart phones and I may be able to get a $100 - $200 on ebay. Not sure it is worth the trouble. I do have a carton of older phones with accessories. I thought I might donate them to my grandkids when they get older to have a garage sale one day. Garage sales I think is a good way to have kids learn about selling stuff to make some spending money. They're a bit too young yet, maybe next year.