I was wondering how do you know how much Prestige you'll get from a purchase if any?
A purchase will give you prestige equal to the amount of gems in the purchase. So a bundle that gives 400 gems, will give 400 prestige points. If additional prestige points are stated, then those are added too. The double "first time" gem purchases will award double prestige.
Emiko's Delivery will only give 100 prestige (the amount of gems on Day 1).
Exception: Calendar purchases do not give prestige even if they give gems. Calendars will only give prestige if explicitly given in a chest, and this will be explicitly stated in the chest contents when you tap the (?) button.