Except for basic sensors the "expensive avionics" have tonnage allocated to them already.
Only the sensor package, not the rest:
HG'22, p19:
All ships must have a bridge that contains basic controls, communications equipment, avionics, scanners, detectors, sensors and other equipment for proper operation of the ship.
There is a limit to the detailed minutiae we have to deal with, thankfully...
E.g. in T5 quarters consist of cabins (of different types and capacity), sufficient freshers (that we have to account for, of different types), separate life support systems (that we have to account for, of different types). Then the total tonnage of the quarters per person (for several categories) has to be accounted for, affecting the gaming mechanics.
Isn't it much simpler to just say one person, one "stateroom", and be free to draw as many cabins and corridors as we wish of roughly the specified tonnage?
Or one "bridge", without bothering to specify exactly how many workstations, communicators, and sensors of exactly what type (modified by TL and capability)?
This is the specification for something like a Scout bridge in T5:

Every line has several decision-points and game mechanical consequences.
Isn't it easier to say that a bridge is X Dt and Y MCr, and be done with it?
The quarters for the same ship:

Isn't it easier to say we need four "staterooms", and let them include all the petty detail?