This is not an endorsement to spend real cash on this game. I am not recommending or saying you are obligated in any way to spend real money on this game. It is perfectly acceptable to be free-to-play.
If, however, you chose to spend money, I wanted to share some tips on the best places to do so.
They added the Gold Brick Mall to the game a little while ago and it gives an additional 5% bonus to your purchases. You buy gold bricks then you can use those to make in game purchases. Adds a level of additional value to your spending.
1) Kimberly for $1 if you haven’t done so already. Even though you already get Kimberly for free you can still get 10 shards for Kimberly for $1 which is a good deal.
2) Second build queue for $2 dollars. While you can use gems for a second build, spending $2 will permanently free up those gems for other uses like trade contracts, secret orders, and the VIP store
This is a reasonable place to stop. $3 for this game is a bargain.
3) Third build queue for $10 is huge for those wanting to level their power faster as it allows you to level your HQ plus the next 2 requirements for the base or just next one plus something else like rss or additional drill grounds and hospitals.
Quality of gaming life spending
4) Fourth build queue and second Tech Center just make life easier for getting VS points and advancing your power and economy. For $20 each they are not a bargain.
Mall purchases
Top right icon with the shopping cart and your gem count. In the mall the only place I’d recommend spending is the Daily Sale for the Bundle for $5 (exception is for ammo discussed below).
Hot Deals
1) During the Bulls Eye event Ammo can be bought directly but a great way to get 6 extra ammo is to go to the mall and spend $1 a day in the Daily Sale rather than the bundle for a total of 30 ammo over 5 days for $5. You need save up ~700 ammo before using them so you can get the 10 Mythic Blueprints.
2) Glittering Market is amazing for having items that can greatly increase your power from base skins to decorations to blueprints and more. You can be patient and save up glittering coins over months or purchase coins.
3) Custom weekly pass is actually a good deal for what you can get. Get drone parts plus whatever you happen to need that week.
Seasonal
During every season you get the option to purchase Seasonal passes and Weekly passes. I do recommend the Seasonal pass. It is a good deal and will allow you power up more quickly throughout the season. The weekly pass is optional expense for those that want to help take down the Stronghold bosses sooner.
Holidays
Large and small spending opportunities during these events. If you are lucky you can even get a base skin for cheap.
Battle Pass
Not a big fan of these myself. While they do grant you items that will increase your power rapidly, most are things you will earn over time for free anyway.