Don't feel bad for not dropping a weeks pay on a helmet.
The safety of a helmet (not helment) is not necessarily proportional to it's cost, excluding the absolute bottom of the barrel low quality helmets. What's more important is how it fits. Arai, shoei, suomy, HJC, KBC, shark, m2r, etc. etc. fit very differently. Unfortunately concerning costs, I have a shoei-shaped head for my street lid. On the dirt my HJC does well.
The reason that more expensive helmets don't always do well is because they can be TOO strong and rigid. To properly protect your melon a helmet is supposed to destruct internally, absorbing those impact forces. That is why the general rule on the street is to replace your lid if you dropped it from 3 feet or higher. Unseen cracks occur under the shell that compromise integrity. I'll admit I've dropped old street helmets, cussed up a storm, and still used them. On the dirt I'm convinced this is not as much of an issue.
Most certainly the more money you spend, the finish, features, and overall quality of a lid improves. Pay attention to how it fits though, not so much with what brand it is. Also keep in mind that a properly fitting helmet may seem a bit tight when new. The padding will compress a bit after a couple riding weekends. Order online for a good deal, but if at all possible go try one on at a nearby shop.
Here's an enlightening article, check it out.
http://www.motorcyclistonline.com/gearbox/motorcycle_helmet_review/index.html