it gets expensive because things that modern ashm has to do can get pretty complex. it can go like this: lauch from tube, pop wings open, start jet engine, get to cruise speed and maintain altitude 2m above water. then get to area guided by gps, but you can't just rely on gps only. then get up, start scanning what's forward with its own radar, when target is found dive to it. or fire up additional terminal rocket engine to get to mach 2 or whatever to make intercept harder (that's what one chinese missile does). or launch 7 or so, get them all to communicate, one flies high above and uses its own radar to find targets and guide the rest of missiles. that one up is visible to radars (above radar horizon) so it can get shot down, but that's no problem because if that happens another missile gets up and does the same thing (that's a soviet ashm, can't remember which one). or missile can stay down and communicate with friendly AWACS or nearby F-35 or something to guide it. there are more options