I actually completely forgot this was a cheap ideas thread after reading some comments and getting carried away. So ignore my other post here (for now), they certainly wernt cheap.
Now for some cheap ideas..
Battery powered sentry guns rather than mains feed. Possibly make them portable but thats probably beyond cheap

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Change the storage square to a container so its a bit more representable of some sort of storage.
Maybe add a global capacity to the storage box and make small and large boxes.
Prison doors or reinforced doors. Can be the same skin as the normal doors but with higher hit points.
Im not sure if this would be cheap or not but a chance on grenade weapon to explode or Molotov's to set the user on fire if the user is hit with bullets. Would be fun with Molotov's

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Ive seen people throw Molotov's on the ground for raiders to run through. Maybe add a fuel item that can be built that has a long burning time. Throwing Molotov's on the ground to set traps would then be a viable option.
Whenever i see people build in caves theirs alot of rubble generated on the floor and looks messy. No one seems to have the time to keep it tidy, i think this could be slowed down a little. Its more obvious when you have carpet layed.
I see theirs a force fire option on people/raiders, well what about a force fire on stuff like batteries so traps can be set causing big explosions.
On that note full batteries should have bigger explosions

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I hope most of these are cheap but i guess it depends how complex your engine is, or if you mean just coding or art aswell.
Edit : something i forgot to mention. Currently you get an alert when raiders attack? How about removing this untill an early warning beakon is placed. This would add surprise attacks. Possibly even remove the warning when they land untill you built a specific item.