I'll play FTP (free to play) games to see what the game strategy is and where the game designers put in the IAP (in app purchases). How far can someone really play without having to buy stuff in the game to keep going?
So far, I've spent $0.99 on Animal Voyage - I bought the intital Bonus Pack that showed up on the left side on my screen. I put off buying it for a few days because I was still learning the game. Once I understood how valuable crystals were, I bought the bonus pack to get the crystals which is a very good real money/crystal ratio compared to what is always offered in the Marketplace - I do recommend spending that $0.99 on the Bonus pack if you are going to play Animal Voyage for more than a few hours.
Unfortunately, I then bew through those crystals on the Mystery Map because I didn't understand that after touching the one free door I was given when I found the map, touching another door would cost me 10 crystals. It says it right there on at the bottom of the screen, but there is not a confirmation pop-up when you touch a door (your crystals just automatically go away) so yeah, no more crystals in a crazy short time.
Right now I'm working on getting my crystal count up to 100 so I can get across the bridge to get the Stegasaurus to complete the Tower of Time.