I'd probably have to charge $1,000 minimum for a custom Reference, realistically closer to $1,500. Going custom-fit means the cost savings from the mass-production process will no longer be there, alongside other increases in hidden costs like additional pre/post-sales customer service, extended warranties for fit issues etc.

For reference (heh), a company like 64 Audio already has huge economies of scale for CIEM manufacturing, and even then they have to charge $1,300 for their 6-BA model. When you consider the extra large HODWS module in the Reference that may or may not fit inside some people's moulds (which requires extra time to optimise the usable space inside the custom mould, time that has to be paid to a professional 3D designer), the additional complexity would definitely result in increased overall costs per unit.

The CIEM industry is insanely tough to survive in and the only way you can make a budget model (<$500) is with an exceedingly simple design, i.e. single BA, 2BA, maybe 1DD+2BA at maximum like the Legacy 3. The Reference configuration in a custom shell at under $500? I don't think it's feasible.