If you don’t care about rapid growth, you don’t have to spend real money on the game. Here’s the cheapest way to do it:
Save your gems for building your city. When you get 50 gems, buy a level 6 building. Building your city will improve what your receive. Later on 2 tier 6 buildings can be combined for monuments.
Go to parties to get charms. Find parties that don’t require you to spend gems, like grab-and-go.
Make sure you collect daily 1 free gem, and free powerups from Fringo, Dragon MahJong, and Lilypond Solitaire.
Play games to finish challenges. Use charms on games. If you want to play without charms, then only play games where a player has an advantage. I know for me that means Blackjack, Dragon Mahjong and Jewels of Osiris.