It's not clear to me it could be very different cost-wise, it's going to rely on one of a limited number of bio-amplifier chips. To be fair OpenBCI have change

It's not clear to me it could be very different cost-wise, it's going to rely on one of a limited number of bio-amplifier chips. To be fair OpenBCI have changed business models a bit; if you're a hobbyist you might go AliExpress but a researcher with a genuine budget will probably pay the OpenBCI "premium" for the same hardware. The other aspect not discussed is that you really want it to be battery powered and wireless if you can (safety reasons), and a raspberry pi device is going to require a lot more battery or have much less runtime.