Here is what I would do in this case. Ask your architect if, in their opinion, the plans have enough detail to get a proper estimate (elevations, structural, exterior finishes, etc.). If not, finish them to that point (called a bid set), if they are at that point, choose three regional builder (selection of the builder is a whole other discussion). These builders should have 1 hour or less of daily commute to your site. Most builders with experience will know that the client is pricing out the plans with several builders and should not be upset about this. If they are, they are not for you. They should be accommodating and interested in your project. I have built for many clients, where I was not the least expensive bid, but I wanted the project and made sure to take good care of my clients. That type of attitude forshadowed the value that my clients were going to get during the construction phase of the project.

Create a specifications list detailing all the finishes you want (to make sure you will be comparing apples to apples). Give those builders the plans and your specifications list and ask for a solid guesstimate within a week. This way you are not wasting too much of your time nor the builder's time and effort. Once you have those figures from the builders (again, make sure they all have the same material to review, even if its not fully done), you can sincerely ask that builder that you like the most (that will provide you the most value, not necessarily the cheapest!!) to provide you with a proper proposal. That proposal should have all the specifications (details of what is included, or budget allowances where there are unknowns), all the exclusions (such as architectural work in your case), and the final price for those plans.

You also want to ask the builder to provide you with several prices for options for particular features (such as hardwood flooring instead of carpeting; stucco instead of siding, etc.). Mainly you want to test if the builder will be reasonable when offering options or changes to the original specification list.

By the way, during the final proposal process a good builder will almost always ask you for clarifications and elaborations on the plans and specifications list to make sure that they are giving you a valid price and not missing any important items and issues. Unless everything is spelled out (which it is not in your case) those builders who don't ask questions, typically will have miscommunication issues with the clients during the project. Of course, not everything can be foreseen, but if the builder is not focused during the proposal stage, I am not sure they will be during the construction stage.

It is not uncommon for good and established builder to get compensated for the final proposal since it takes a lot of time and effort. When I ask for compensation, I typically credit it back to the contract if I am awarded the job.

Discuss the changes that you want to make on your current plans and what those would cost.

Once you are comfortable and have done your due diligence on the builder, get areal estate attorney to review any contract and make sure you and the builder are on the same page as far as all the terms. Many people skip this stage for some reason and then get in trouble!

Good luck with your project.