I imagine subsonic AShMs also offer a certain amount of flexibility in terms of routing, man-in-loop guidance (thinking of Tomahawk here), and coordinating the arrival time of multiple weapons over the radar horizon of the target and/or its escorts. I also wonder how they compare to super/hypersonic weapons in terms of cost, all else being equal. The US, for example, has the potential to repeatedly deliver enormous volleys of syncronised subsonic AShMs via its vast tactical and (especially) strategic aviation fleets. For example a single B1 can apparently carry 24(!) LRASMs. That is a disgustingly large number of stealthy cruise missiles on a single aircraft.
I suspect quantity has a quality all of its own when it comes to saturating point defences...
Evaluating the costs of modern supersonic vs. high subsonic AShM IMO would not be difficult, but I do not believe that it would be particularly informative.
The modern supersonic AShM tends to be of Indian or Russian manufacture which tends to cost less than US/European manufacturing. With the US/Europe, and then mainland China being principally users of high subsonic AShM, one could look at export contracts to try and figure out a per missile cost for both sub and supersonic AShM, that might only provide cost information.
It would not really provide any information on which missile was more effective, and it also would not include the costs or supporting elements of the battlesystems used to achieve an effect with the AShM.
For instance, there would need to be an MPA and/or ISR asset to detect the potential targets for the missile strike and have that asset either launch the strike, or cue other assets to launch and a datalink to relay targeting info.
Given the size of most of the supersonic AShM, it appears that it would take a large number of launching platforms to achieve a significant number of inbounds. Looking at what the Soviet maritime strike doctrine from the 1980's had been, this was to be achieved by regiments of Backfires launching against a USN CBG. At this point, there are only a handful of nations which might be able to operate that number of aircraft at once, never mind directing that many against a single target or task force.