I wrote a sidebox module for product filter by attribute, and price range. I looked all over the places and couldn't find something similar, so ending writing my own. It seems to be working, but only tested using zen-cart v1.37.
Don't forget to read the README file first. I tried to sumit to zen-cart.org. But the file didn't get submitted (don't know why).
Anyways, Here's the download page in my blog. Zen Cart Product Filter by Attribute, Price RangeZen Cart Product Filter by Attribute, Price Range. I wish zen-cart can give me a place to host the file. I've already tried to submit to the download page, but it failed to upload.