The B650 and B650E southbridges are probably the same but that doesn't mean the boards are the same. PCIe 5.0 has tighter tolerances which requires more expensive PCBs. Motherboard vendors don't future-proof low-end products when they can upsell it to you instead.
The board already supports 5.0, and the stack up requirements for PCIE 4.0/5.0 haven't notably changed. So, I don't think this is a PCB cost question, more of a design time question.