You know 'BOGO' stands for 'buy one get one' with no mention of 'free', right? A BOGO deal isn't as complicated as people try to make it out to be.
You upgrade a line (buy) and add another (get). The 'get' phone is credited a certain amount over the course of 24 months. The promotion will state how much -- Verizon will have the details online. Typically this is around $750 with some promos being enough to cover the cost of the entire device.
For phones that cost more than $750 in this example, you're responsible for the difference. If the phone is 1k and the BOGO deal gets $750 off, that leaves you responsible for $250 over 24 months.
BOGO deals aren't that complicated to figure out. You're not getting all the money off up front because it's a bad business move thanks to fraud being a well known issue. This is why credits get applied monthly over that time frame. If you owned a major cell phone company, would *you* casually gave away nearly a grand up front for millions of customers? No, you wouldn't. That wouldn't make any business sense.