Can I sense filter replacement need based on filter pressure drop?
Since the purpose of replacing filters in the first place is to restore airflow which has been diminished by reduced filter porosity, it would be a better approach to use a differential sensor to monitor airflow, rather than a differential across the filter(s)

In a ‘passive’ system with no blower control, the filters are simply replaced when the flow has reached some minimum acceptable level.

In an active system, energy to the blower is regulated in a control loop.
The desired operating volumetric flow is first established, and this forms the reference for two control set points for the loop.
When the flow reaches the lower set point energy to the motor is increased to restore specified airflow, and decreased when the upper set point is reached.
Filter replacement is indicated when the blower can no longer sustain the required flow. 

An example of how a differential sensor is applied in deriving volumetric flow can be found here:
http://www.bapihvac.com/CatalogPDFs/I_App_Notes/Determining_Air_Flow_CFM.pdf

I have just completed a design applying this principal in extending the service life of HEPA filters in a forensic laboratory environment.

I also used the Freescale MPX2010DP, coupled to a PIC16F818, which has just enough pins to drive the 4 seven segment display digits, scan three buttons (START/STOP,  and set point UP, and set point DOWN) and implement PWM control on the motor.
The sensor requires to  feed into an instrumentation amplifier (fabricated from the ubiquitous LM324) as the full scale voltage output differential is only 0.4V, and the ADC on the microprocessor requires an order of magnitude greater than this to realise adequate resolution.

The two air duct  probes, one for total pressure and one for static pressure, which connect to the pressure and vacuum sides of the sensor respectively could easily be fabricated DIY  from thin copper, aluminium or other soft metal tubing, or they may be purchased online for very little.

The cost of materials for this type of project would be trivial, the question is whether one would enjoy and derive satisfaction from the substantial effort it will require.