The sensor is in front of the bottle for windshield wiper fluid.
It tells the ECU your altitude. Used to control boost and turn
off some things at altitude (like controlling fuel injection time
at altitude when starting).

I did not see a price for it listed on alldatadiy.com (you pay $25
a year and get access to Volvo technical info).

Here is a list of sensors on a 98 Volvo (as you can see, lots of em):


Computers and Control Systems Components:

A/C Signal .
Accelerometer .
Air Flow Meter/Sensor
Air Temperature Sensor .
Barometric Pressure Sensor .
Camshaft Position Sensor
Central Electronics Box .
Coolant Temperature Sensor
Crankshaft Position Sensor .
Data Link Connector .
Engine Control Module
Engine Speed Sensor .
Fuel Level Sensor .
Fuel Tank Pressure Sensor .
Hall Effect Sensor .
Idle Speed/Throttle Actuator - Electronic
Knock Sensor
Main Relay (Computer/Fuel System) .
Manifold Pressure/Vacuum Sensor .
Oxygen Sensor
Throttle Position Sensor
Transmission Position Switch/Sensor .
Transmission Temperature Sensor .
Vehicle Speed Sensor .
Vertical Acceleration Sensor