Hopping is a flat price based on the number of total days for the ticket. Hopping for a 3-day ticket costs (I think) $80 -- whether you hop 1 day, 2 days, all 3 days or even none. It doesn't get pro-rated for a partially-used ticket.
It is a "set" charge, roughly $80, to add it for the length of ticket dates. Whether it's one day left or however many days are left.
That's why if you are really considering hopping at the end of your ticket length, purchase for the whole length gives you a lot of flexibility
Lots of great information but I would go with a modified version that @Mouse Tracker suggest about early is better than later.
I would wait until you enter your first park, and then if you still want to add the Park Hopper option add it before you leave that first park. The is a great compromise.
Price depends on the length of ticket:
$80 to add hopping to a 2- or 3-day base ticket.
$90 for a 4-day or longer ticket.
For a 1-day ticket it's variable but roughly $60 over the highest park ticket price that day.
But always based on the length of the ticket, not how many days are left or how many days you might plan to hop.