GPS/WAAS equipment cost is just a fraction of the total cost. What's expensive is the actual cost to retrofit each and every individual aircraft. It's not like you one simply unplug old equipment and swap in new ones.
You have to include the costs to the airport infrastructure, such as High intensity runway and approach lighting, centre-line lighting, not counting imbedding all these in-pavement and such issues such as CAT II and CAT III hold short lines, especially if taxiways have to be redesigned for that. Plus, there has to be increased lighting for taxiways, such as the 'greens' centreline taxiways. And for airports that have never had CAT II/III systems, an huge stand-by generator to power the whole thing. Then, once an airport has imbedded runway and taxiway lighting, snowblough blades have to be converted from steel to plastic, so as to not rip them out during snow clearing operations.
In the case of Halifax, the cost to the airport authority for the improved infrastructure to accommodate a CAT I for 06 was similar to the cost of the ILS itself to Nav Canada.