It's not about the ammo cost. The LCR in .22 mag would offer more shots and lower recoil with ammo that I believe is fully capable of ending a threat.
If you think, as I do, that "shot placement is king, penetration is queen, and all the rest doesn't mean jack", then the .22 magnum is a valid choice for a defensive handgun.
Given the choice, I would purchase and carry the LCR-22M over an LCR in .38 Special. In this modern world you may face multiple attackers and 5 shots starts to seem a little meager. No recoil, cheap and painless practice makes me think I will place the 8 shots accurately.
When you consider the millions of new shooters, many of them women, and also the many older shooters who are starting to have wrist pain and other issues, I think there's certainly a larger market for the .22 magnum as a defensive round.