InnerFidelity, and probably others, have done a write-up on the Beyers that come at different impedance levels. As far as I'm aware they're the only manufacturer that actually does variable impedance like that - for example there's not a 32 ohm HD 600 or 600 ohm K701. So basically this question only applies to Beyerdynamic in a meaningful way, and they have their reasons for making variations on their products (usually relevant for professional users). You can read InnerFidelity's write-up here:
http://www.innerfidelity.com/content/comparison-beyerdynamic-dt-880-32-ohm-dt-880-250-ohm-and-dt-880-600-ohm-headphones
As far as the bigger question you may be asking "does impedance impact sound quality" - not in any way that can be appreciably discussed, again except for Beyerdynamic. For example you cannot say "oh, well because Sennheiser HD 600 is higher impedance, it's better/worse than Grado RS-1 which is low impedance!" - that just wouldn't work. Don't buy into arguments that "low impedance is low quality" or "low impedance is only for portability" either - there are plenty of high end cans (yes including Summit-Fi stuff) that are low impedance. Basically the only reason you should worry about impedance (again, except for Beyerdynamic) is for compatibility (and I'll add to this, impedance is NOT a linear metric of "hard to drive"-ness or anything of that sort), since you don't really get a choice in the high/low debate for the vast majority of headphones on the market.
Amazon's pricing is itself fickle - I've seen the 600ohm Beyers going as the cheapest of the set at times in the past, or the 250ohm "Pro" being the cheapest, etc. It moves around.
