There are several really cheap routers that cost about the same or less than a Pi plus power plus card. OpenWRT is preconfigured open source router software for your router or Pi or several other small computers. You can have your cheap router and, if you do not like the router software, install a free open replacement.
There are several posts about the difficulty of configuring Linux for a router. You have to work hard to put in place all the controls and security. I recommend premade routers for people who are not training to work in the network server/router area.