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We can hack the Android version to drop the price of elements to near-free.
To do this:
java -jar testsign.jar hacked.zip ready-to-install.apk
All the elements should now cost just 3 mana.
There is a guide to hack the iPhone version by editing rewards.plist, a file which exists in the Android version too, but making the change suggested in that guide had no effect, which is why I hit the shop.plist instead. Perhaps they left that file as a red herring?
Also, you may be tempted to try to edit the player profile.dat directly to change your mana, and you can find the binary number to do it (offset 0x94), but the file is hashed with a 160-bit hash (probably SHA-1), and the hash is salted or something, because I couldn't figure out how to re-sign it.