Boosts will be available in several different durations, with the longest duration corresponding to the greatest overall bonus. Players will have improved drop rates, health regeneration, and battle rewards as well. Prices have not yet been announced, but the goal is for the boost days to be affordable. Players with subscriptions will also see their remaining subscription days converted into boost days, allowing everyone currently playing to reap the benefits early.
[Update: The team has just posted another chart with all of the veteran rewards that will be distributed to current subbers on July 9th.]
[Thanks to Avaera for the tip!]