Yeah, with the new model, the 'base' phone plans have dropped (at least in my case and with my carrier, T-mobile).
Everything is modular. Base plan, then add the phone (retail price / 24 months). The 'base plan' is cheaper then it was several years ago with the subsidized plans, and includes more (unlimited min/texts/data in my case).
I'm guessing that the 'trade-in' thing basically is telling you that if you return the device during the 2-yr cycle, it will relieve you of paying the balance on the installment plan. In my own experience, these devices in the first year more or less depreciate at the rate at which you're paying. In other words, if you opt for a $600 device on day one and are paying monthly installments, after 12 months (mid-way through a 2 year payment plan), the device's street value is roughly half (to be fair, it's probably still worth a little more .. maybe $350), but when you consider the inconvenience of having to list/sell and ship it, I'd rather eat the $50 and move on.
The rate of depreciation slows considerably after a year, so if you tend to keep your phones a long time and/or want to keep it as a backup, you're better off paying through 24 months and keeping it, since after a full 2 years, it's likely to still be worth about 25-35% of the original price.
Obviously, some people lean more towards always having a newer device and others don't mind having a paid off phone and upgrading on a 4-5 year cycle.