I may soon be upgrading to 2.5Gbps internet, however all the routers that support said speed seem to be expensive. Is there any that cost less than $100?
It'll be hard to find one at that price. You may find something with a 2.5G NIC but whether or not it will actually route at line speed is highly unlikely.
Your best bet to keep prices low is to add a 2.5G NIC to an old PC. Even that may not work