"But the whole point of what I’m doing here is trying to gauge approximately what the cost will be before I commit any design money."

That's a perfectly reasonable objective. Nobody wants to invest in design of a project that won't get built. The problem is that any estimate needs to be based on something--either plans or a similar, recently completed project for which construction costs are known. In an environment where construction costs are relatively stable, the "comparable project" is a good approach. Unfortunately, we're not in a "stable" cost environment.

I suggest you have your architect/designer produce a concept floor plan and elevations--in lieu of fully detailed plans and elevations--and use them to get a reasonably accurate estimate from your builder. With that information you can decide whether to proceed to the detailed design phase or reworking the concept plans to reduce construction cost or deferring the project. You'll spend some money on design, but not as much as you would spend to produce a fully detailed set of plans that could be used for competitive bidding and permitting.