The gold grind is SUPER slow without the bonus.
Even with the $$$-paid bonus the grind is quite slow.
The best way to earn cash is actually grinding the first level repeatedly then take the gold instead of continuing on (with the gold bonus).
If you want to spend money but RL circumstances do not allow it:
SIgn up for google rewards https://play.google.com/store/apps/details?id=com.google.android.apps.paidtasks&hl=en_US
You get money for answering questions about your shipping, travel, and browsing history,.
Every penny of the ~$15 I spent on this game came from the survey rewards.
(That is not an affiliate link and IO get paid nothing for sharing it. It's just a link to the app store and I do not work for google. Its not spam lol)
The fastest way to earn gold is to beat the first level repeatedly then call it quits for the bonus gold.
Simply grind the first level repeatedly.
The first map is the best (easiest to guarantee a finished map) but I also did this to the next few maps for unlocks at the same time as gold (if you have most/all of the first maps unlocks).
The booster upgrade helps a LOT.
If you don't have RL cash, sign up for "Google Rewards", you get free google play money for answering surveys, taking pics of receipts, and answering locations based questions
https://play.google.com/store/apps/details?id=com.google.android.apps.paidtasks&hl=en_US&gl=US(Not an affiliate link and I get nothing for you sign up, just a link to the google play store)