Ever try outdoor motion sensors?

[IVB]

Why not just keep the existing light fixtures and remove the sensor.  Plugs for the holes are probably $2.
 
So the cost is
LV motion sensor     $90   (your number, think you could get it slightly cheaper)
RIB                           $15
Plugs                        $  2  
 
      Total                   $107
Almost the same cost as a new motion light.  (the sensor is cheaper and likely the only thing that needs to be replaced)
 
Think you said you already had the elk relays and such to run through the HA system.  The only pain may be where to put the RIB - may require some HV wire runs.

Dumb question, what's a RIB?

And yes Pete, I realize I'm opening myself up for abuse there

 

[JimS]

Relay in a box.  They have a number of models.  Their basic models have attractive pricing.  Their delay and other models not so much (IMHO).  But with a HA system you just need a basic model.  It allows switching line voltage and avoids all sorts of issues with NEC, ect. 
 
For this use the RIBU1C looks ok.  Low voltage input is 10-30V ac or dc.  About $15 on Amazon.
 
http://www.functionaldevices.com/pdf/datasheets/RIBU1C.pdf