I shop for old helmets all the time at thrift stores. There are many good old helmets out there. Unless it's been in a wreck, which is usually obvious, or it's had UV or some other kind of damage to the shell, which you can test for by squeezing the helmet - if the shell itself crackles or makes other unusual noises when squeezed it's toast - old helmets are fine, at the fraction of the cost of a new one. So, IMO, if they are in good shape, you should recycle them, if they are worn out or otherwise damaged, you should toss them.