What is the payload for JESD204B at 12.5Gbps line rate?
The payload is the data before 8b10b encoding.
Therefore at 12.5Gbps the user payload is 8/10*12.5Gbit = 1.25Gbytes/s (per lane).
No other protocol overhead applies to JESD204, so the payload is 1.25Gbytes/s.
(In Ethernet applications, for example, there are idle characters and header bytes which reduce the payload below 8/10ths of the line rate).