Ok, some terminology:
Sensor: the thingie that generates data (audio, video, etc)
Sensor Array: a group of Sensors working in tandem
Sensor Housing: the thing you put a Sensor Array in
So, some examples.
1) let's say you have a shadowrun that requires you transit through a toxic wasteland. Ergo, you're going to want some kind of hand-held scanner that detects both radiation and toxic vapors in the air. So for starters, you know you want it all to fit into a hand-held scanner. This physical and empty shell with no sensors in it costs 100 nuyen per point of capacity, with a maximum of 3 capacity. Each sensor function only consumes 1 capacity, so a geiger counter and an atmospheric sensor each require 1 for a total of 2 capacity. Ergo, a capacity 2 hand-held sensor meets your needs: 200 nuyen so far for the Sensor Housing. Of course those two sensors have to be acquired. The maximum sensor rating you can put in a hand-held sensor housing is 3. Since sensor functions only consume 1 capacity no matter what their rating is, you can either buy max or save money. 2 rating 3 sensors costs 300 nuyen each, and since your life is on the line why skimp on a couple hundred nuyen, neh? A hand-held housing with 2 rating 3 sensors costs 200+300+300=800 nuyen.
2) let's say you've acquired some kind of supervillain ego and are building a fortified lair in the middle of the wilderness. You want to install some kind of early warning radar system to alert you when someone's flying an aircraft in the direction of your super lair, and money is no object. The maximum rating Radar (sensor) available in the CRB is rating 8. This radar costs a laughably low 800 nuyen, but them's the rules*. Of course, you can't just put a rating 8 sensor in a hand-held housing! Per the Sensor Packages chart on pg 277, you're going to need to obtain or construct an entire building to house your 800 nuyen Rating 8 Radar. It'll doubtlessly raise the total cost of your radar detection site rather significantly, but there's no rules on how much it costs to buy or build an entire building. GM fiat here!
of course, once you DO have/acquire your building to house your giant radar dish in/on, since you have such a huge Sensor Housing you could if you want put an array in that building to join the Radar system. Or just leave it as a "single sensor housing".
*EDIT: Ok, actually, RADAR
isn't in the rules as an available sensor function. Of course a GM could fiat something, but in this case I'd strongly recommend "tweaking" the rating x 100 nuyen cost a bit
Redoing example 2, as something that is actually practical:
2) let's say you maintain a safehouse, and you want to install a great MAD scanner in a strategic chokepoint. Maximum rating available is 8, for 800 nuyen. Something that capable cannot be housed in something as small as a RFID tag hidden on the wall, or even in a hand-held housing you issue to a guard or such. It's going to have to be installed IN the building structure itself, necessitating whatever renovations/costs deemed appropriate because again, there's no rules for general contractor work.