You want a sophisticated firewall but don't have the money required. The good news is that there is an open source project that has created software that can replace the original software of many low-end firewalls. The resulting firewall has the features of a unit ten times more expensive.
No only are the features better, but you can even turn up the signal strength. Below is a link to the site that explains the features and provides a list of all the models that can be upgraded -- you may have one already.
What is "DD-WRT"? - WRT Wiki: "Free Firewall upgrade"