I use OPNsense on a reasonably cheap mini-PC with 6 ethernet ports that I bought from AliExpress. You can do a lot with OPNsense. For wifi I could have added a wireless card, but to avoid any potential driver issues instead I just plug in a wireless router set to Access Point mode.
How much this kind of setup costs depends on what kind of hardware you buy. If you stick to the minimum spec for OPNsense and buy a cheap wifi card or a used wifi router from eBay, you won’t need to spend too much.