Just on the topic of Mythril, if this is someone's first game, this is something to think about:
FFRK generates 1 Mythril per 5 days, which equates to about 1 rare draw per month.
Brave Frontier ensures 1 gem per week and, after 25 days of consistent login, offers an approximately 1 in 9 chance of getting a gem per day (don't know how true that is in practice, but it's presented as 1 in 9 chance).
Puzzle and Dragons has no guaranteed stone reward, but does have events every so often that will provide 5-10 stones over the course of one or two weeks.
Fable Age provided gems once a week as well.
So, all in all, it's fairly standard.
Some typical pieces of advice with IAP:
1) Don't use it to refill stamina, ever.
2) Don't use it to retry dungeons after you've died. You died once, you're likely to die again.
2) Don't use it to expand box space unless completely necessary. Box space is a luxury, and only once you've started filling up rows with items/cards you're going to keep should you expand.
Edit: And yes, as Pixy says, Terra Battle gives 5 energy per week.
Terra Battle is awesome.