It has its downsides I believe but Banana Pi R4 + OpenWRT can be a good combo.I dump on the AI companies every day. Even Supermicro sold us out, the lower end of the spectrum no longer exists, I had the eshop give me a $4000+ suggestion for a firewall, and that was before memory or storage was added, just a board, chassis, and the lowest processor in the series. All they really want to sell is now the giant AI servers that make them so much profit.
If you want something cheap and don't have unbuffered DDR4 already (with the given Prices), a Supermicro X10SLL-F/X10SLM-F + Xeon E3 v3 + 16-32GB DDR3 unbuffered ECC + a Quad Port Intel i350 NIC (if desired) + a dual Port Intel X710-DA2 / Mellanox ConnectX-4 LX is still great to this Day. Probably 150-250 EUR including everything.
Or as @kapone suggested whatever is cheap out there if you only need to do "Firewall".