If you're looking for something cheaper than a Pi try finding used thin clients, which are basically x86 industrial PCs. With a managed switch behind it you can

If you're looking for something cheaper than a Pi try finding used thin clients, which are basically x86 industrial PCs. With a managed switch behind it you can use seperate VLANs so that you can run pfsense with both WAN and LAN on a single NIC. Obviously this will cause reduced speed on the WAN-LAN interface due to the single shared NIC, but if you're looking at a Pi anyways the networking performance of the thin client may be adequate for your needs.