Say a structure has 1000 HP (like the weird Large Collidable Structures for walls and lookout posts used to create ‘complexes’ in missions). If you are using Iron Charge S, the most basic ammo in the game, which is also the cheapest, with level 3 in all the support skills for the ship hull and gunnery for hybrid turrets, each ammo does something like 10 damage (pretty sure it is less than that but I don’t feel like booting up PYFA for this). That means it takes 100 ammo to kill the structure. That ammo costs on average 6.5 isk each - so 650 ISK. The structure then drops 5 holoreels (assuming it drops anything at all), worth about 275 ISK each. Yay, you made 725 ISK! So you shoot the next building… which has 10,000 or 50,000 or 250,000 HP - and it still drops 5 holoreels. Maybe a few tourists, too. Still loot worth 5000 ISK or less total in most cases - and the more HP the structure has, the more expensive it is to kill. Using better ammo? It also costs more per shot.
Seriously, there are I think maybe five missions that have structures that reliably drop loot worth more than the cost of the ammo used to nuke them. I believe all are level 3 missions or higher.