AIA Small Project Design (SPD) Knowledge Community supports, celebrates, and promotes small projects by engaging designers and the public.
Not sure how the original poster was selected for the task in question, but, be aware that:1. The design-build business for churches and private schools is very lucrative for the Builder if they lead the team. At times the sales approach is similar to used cars.2. Perhaps the first group is trying to break in to this business, or is busy with other work, and didn't see what was happening with costs.3, Another factor in working with amateur "building committees" is that there can be a few members who are retired contractors. Others on the committee defer to them, and to their memories of what prices used to be, how every other contractor was out to gouge owners, etc. and have no real idea of "modern" construction concerns, material costs, labor availability, etc.
On the other hand, one of my first bosses said that working as member of a community organization (for free) was a great way to learn how other people think, what was persuasive, and to show your skills off to other people who might hire you in the future.Based on their experience so far, I think you could "sell" them on their need to hire an independent cost estimator with experience in the type of construction at hand. Clearly they have a need for a lot of hand-holding and guidance. Don't expect to do more than break-even on a job like this, and keep your eyes open to the group dynamic.