If you check the Slickdeals site, there's a ASUS router that is compatible with Tomato and DD-WRT firmwares. Tomato is lighter on features, but also lighter on the router's resources, DD-WRT is a bit more feature full.

Think it's around $50 before a rebate which brings it down around $20. Don't know much else about it, so you're on your own to figure that out.

You can also poke around on ebay for a V1.0-1.2 Linksys WRT54GS router which had 32MB of RAM, making it quite the little workhorse for dealing with large numbers of connections... Assuming you install Tomato or DD-WRT on it.