I actually just got sick of my old costco chair too, its lasted me ~8 years now
Amazon.com - Alera Elusion Series Mesh High-Back Multifunction Chair, Black - Task Chairs
This is the one i got for a sibling as a gift (lame gift, but they were going away to grad school so I figured they'd use it a lot). I liked it because my legs actually fit onto it -- some of the cheaper chairs (i.e. ikea) have such a short length that half your thigh hangs off if you're 5'9+
or, you could scour craigslist for a nice used herman miller in the $200-$400 price range -- those chairs are
but I'm not sure if you can justify the cost (I couldn't)