I have a Troxel Sierra and while it's definitely nothing gorgeous or suitable to wear while showing, it's light, breathable and after a few weeks of getting used to it, I no longer notice that I have it on.
The lightest, most breathable, helmet I've found that fits my small head very comfortably is the Ovation schooler. It's not "cheap" at $60 but I've found it to be well worth the money since I actually enjoy wearing it!
Troxels are even lower in cost in some instances. I'm drawing a blank on what exactly IRH stands for but their good helmets as well. Posted via Mobile Device
I board my little WP horse at a predominately eventing & dressage barn and most have helmets which are several hundred out of my budget, so I don't have in-person recs. I've seen the Ovation Deluxe Schooler and several Troxel models for around $60, which is reasonable enough to try, I suppose.
Ovation! I love my ovation helmet. Super light weight and very "cool" as it is well vented. I would stay away from Troxel. The harness on them is known to come loose while you are riding. We have banned them in our school due to a few incidents now and I keep hearing more and more horror stories. My students troxel flew off her head before she even hit the ground and I had checked it before she got on.
I've tried a few of these lightweight helmets and they don't seem really comfortable, the Ovation my husband has is about the best but I've gone back to my UK Charles Owen skull cap, it really isn't that hotter and fits my head better