Unless you have more money than you know what to do with or are a completest and have to have every unit there are smart ways to spend gold (or not)
1. LTQ - currently not worth it. The latest LTQ has a 50K unit with no bonus and requires a vault of gold to complete (as have most of the recent LTQS). Better to wait for 24 hour LTQ (which in the past had awesome units with great bonuses). If it has no bonus skip it. If you are over Level 150 you may want to go as far as you can free. Gree has made these that even with prestaging you need at least a truckload and most likely a vault.
2. Box events - not worth it. You can drop vault after vault and not get the unit or prize.
3. WD event. The best way is to get in with a good faction and find out what they expect from you as far as WD points. If you are a super active player you can get away with less gold and still meet the point requirements. If you are in a streak faction which waits until final minute or 2 to attack you may need to cough up substantial gold to keep the streak alive.
4. FLTQ event. Far and away the best value for Gold and you may not need to spend any or very little for a great reward. The inaugural boss event which just ended handed out great units with no gold necessary except on the 15 minute 100 Boss which needed maybe 50 gold/player
5. Medal events. You need to complete all of the events to get the required amount of medals for a mega unit. Necessary gold for this is impossible to calculate.
6. Boss event - if you are under level 150 you can go pretty far in this event and some gold can get you an excellent unit. If you are over 150 forget it, you spend a 2-5 vaults to complete the event as the bosses keep getting stronger each event. Keep you level as low as you can for this.