Many reasons have already been given for why you shouldn’t sign up for cryonics. And while “It wouldn’t work” and “You can spend that money better” are still the best arguments, I would like to add a minor one to the pile.
Not only could the money that is spend on cryonics be of better use, so could your body. Donating organs being the obvious one, but even if you (for whatever reason) don’t want to do that, you can still give it up for scientific studies. Or if you don’t want to do that, it could be a practice body for medical students. Or if you don’t want to do that it could be used by necrophiles as a sex-toy or by farmers as fertilizer.
Again the other two are still the main arguments against cryonics, this is just a minor one.