Being trained in both helmet fitting and saddle fitting, I can say that you can get great helmets - so long as they fit! - for $300 or less, and I wouldn't spend more than $500 on a helmet. Just because a fashionable helmet costs $900 does not actually mean it is as safe as a $300. For saddles, generally the more expensive it is means better quality, and (hopefully) better fit. They're so important to the rider's seat and overall balance, forwardness and muscular development of horses that it is something I would not skimp on.
I'm willing to pay more for quality, safety, and sustainability. A good fitting helmet, quality leather and stitching in tack, that sort of thing 😁 rather buy a more expensive, good quality bridle than a cheap one with poor leather that breaks after a year.