An alarm with nothing in front of it never rings, so budget the pair, not the part. The trigger you choose decides the real bill:
a seismic sensor hears explosives and takes a second piece of tech trash, so the classic raid alarm costs a C4's worth of circuitry;
a pressure pad just inside the loot room takes no tech trash, only a gear and a spring;
an HBHF sensor is frags only, but it may ping you whenever your own team walks past.
Research the alarm on the same trip as its trigger. Both sit at the Level 2 bench, and an alarm without a trigger is a box ornament.
When tech trash runs short, this pair still comes before a second sight or a laser. A holosight wins one fight; the alarm saves the loot room you would otherwise lose while offline.
