I'm not really sure that's true. I'll bet there's things you get by spending more (weight, comfort maybe), but as far as I can tell, there are only two safety standards -- whatever you call the the one that all helmets have to meet to be sold legally, and MIPS, a newer standard about rotational impacts (ie your head sliding on the ground when you crash). I just bought one of the cheapest (road) helmets I could find that meets both standards (the specialized align II for $50) and as far as I can tell in actual crash tests (
https://helmet.beam.vt.edu/bicycle-helmet-ratings.html) it's up there with the expensive ones. I actually like the look better than a lot of the pricier ones too, it's pretty subdued. Maybe the real lancers will know what it is and flag me as a kook but whatever.
Otherwise yeah, definitely agree with not buying used. It could be totally useless and you'd have no idea.