Firewall Appliance allowing white list - blocking all other traffic

Are you aware of any firewall appliances under $300 that would block all internet traffic except to sites that are allowed via a white list?  We have a chain of stores with one or two computers in each store that have internet access so that employees can access a web site to clock in, and also connect to the home office through a VPN.  I would like to have an appliance that I could drop in that would restrict access instead of setting up another computer in the store to serve as a proxy as we are already cramped in regards to space.  I have seen this feature in some of the higher end firewalls, but the only one that I found in a smaller appliance (Hotbrick) has been discontinued.