Look in K2SO aka Escape from Jedha it had multi tiers with rewards.
You can play each Tier that you can beat each day 1 time for free during duration of event.
Additional plays can be purchased in each Tier Daily, but are not needed unless you want the character faster, or cannot complete every level evety day to 7☆ it.
Keep in mind crystal refresh prices remain the same on each and each tier has a daily limit of like 10 refreshes. So don't wait until the last day to purchase what you need to 7☆
To get the character 100% 7☆ for free you must complete every tier every day during the event.