It doesn't have to cost a fortune to have a "Plan B" in place.
It's also usually proportionate to the size of the tank...
It's also usually after somebody learns the hard way, that a few dollars could have prevented catastrophic losses.
We all know this isn't a cheap hobby. What we collectively can tend to overlook, is the priority of hobby-related purchases.
I recently got caught with the proverbial pants down when we experienced a 27 hour power failure and our generator was not in working order. Thankfully, a neighbour in the plaza came to my rescue with a loaner - my generator is now REPAIRED and that won't fall off my radar again.
Running to the LFS stops being an option if your pump dies at 10:30 at night... I've been called at home - and out of bed, in wee hours to help somebody - and happy to do so but if you can't reach your "person"....
And overnighting something might be a bit too late....and it's way more expensive than having a spare at the ready.
Powerheads will keep a tank limping along until a return pump or part can be sourced.
Sometimes, emergency purchases may end up costing more, than having a spare already - an impeller is cheaper than a whole pump if an impeller isn't available. A decent used pump as a spare is cheaper than a new one bought in haste.
For small tanks, a battery powered air pump is about $25 plus the batteries... and that can be the difference between survival and failure. Use 2 on a medium-sized tank.
When you replace your lamps - keep the old ones if they still light - an old bulb is better than no bulb.
Have access to a container of some type that can contain your stuff in the event of a tank leak - that is a rare occurrence, it's more important to have water-circulating devices and heaters and alternate light sources (IMO in that order).
I'll go one further... it's equally important to have a quarantine of some sort, and a few basic necessities for that as well as an "emergency kit" for your tank. Prime or Alpha for a variety of uses (dechlorinating tap water if an emergency water change is needed and not enough RO is available, binding ammonia/nitrite, etc.), a PolyFilter to remove a variety of toxins and/or medications, and maybe a basic medication or two. Again, doesn't have to cost a fortune, but a small wise investment here and there can save many, many dollars - and lives - and grief, in the long haul.
An ounce of prevention is most definitely worth a pound of cure. Not having at least a minimal back-up kit, is like driving a car without a spare tire.
Jenn