are open headphones are much more expensive than their closed counterparts?
Open headphones aren't necessarily harder to make; on top of @meltie's reply, it can be the reverse. You'd have to make the dampening and chamber size in a closed headphone a lot better than just having a grill at most behind it upon which the soundwaves can bounce off (any driver produces sound on both sides of the diaphragm); in other words, the exact opposite of what makes a Grado inexpensive.
The pricing trend all comes down to marketing - if you were a manufacturer hoping to sell a sub-$100 headphone, what are people likely to require from it, or what kind of people are most likely to buy them? Off the top of my head:
1) Portability - most likely someone looking to hook it up to an iPod or other more mainstream players (post-smartphone that's likely to be a tablet also)
-This requires an efficient, closed cup design - with 15mW enough to get it loud, closed to isolate noise and make the most out of that 15mW, earpad-cup size however
might be supra-aural to save space at the cost of better isolation
2) Cheap and/or rugged construction - goes with portability
- It's either a throw-away, or tough enough (actually
and cosmetically resistant to relative abuse)
3) "Style" - this is not exclusive to $300, rapper-endorsed headphones; you can't put Hello Kitty's face on a grill, right? But you can on a closed headphone cup, and some kid will ask Mommy to buy a $39 pair because it's cute. Hell, my Mom almost got ME one when it was going for $12 each, because she liked Hello Kitty since it first came out. I had to explain that I'd rather spend my money feeding a real cat than loving a cartoon cat while scolding her then-9yearold son for bringing in a dusty kitten he found in the bushes.
In both cases you can reach out to a wider market than just audiophiles, or in the case of cheaper, semi-open monitoring headphones like AKG's K66/77/etc, pros (including amateurs and students - either way it's part of their job now or in the future). In any case, if you don't mind the disadvantages of an open cup design, there's always the SR60 and SR80. Get one used and tune it to the sound you want, like with different pads, maybe the cups too. I've walked around campus with them after having used the bulkier K66 and a cheap $25 Philips before that, and personally people calling it "retro" (but evoking turntables) feels better than "retro" evoking Ben Stiller jogging.
Or for someone in Asia, at least one Super Sentai series, which is what I got a lot more of.