I've used a number of different markers, and really, between quality and ease of use, I've found that Copics are worth the extra couple dollars. And the blend is much better than anything I've been able to achieve with Prismas or Trias (and yes, Trias are refillable as well as Copics, but I don't like the inks or the color choices in general). I've seen a few other brands out there, but one of the things to keep in mind is accessibility (eg, do I have to have to order it online and wait a week or can I just run down to the store and pick up another?)
If you're in the Eugene/Springfield area, the UofO bookstore has them for $5/each, and 20% off if you buy more than 12 (which makes it more like $4 for each.) If you're near a college campus at all, check with them. They'll likely have them below list.