With the current event going on, you should spend your rubies resetting Halloween missions to get the best ones, and finishing them early whenever it fits your schedule better. Several of the missions give multipliers, and spending some rubies for mission rerolls can give you multipliers much cheaper than buying it in the store.
Your greatest boost will come with the late game decorations that give multipliers to individual girls that will rival and possibly outgrow your general multiplier bonus, so buying double shares to get to the end game quickly is also a very viable strategy.