In obtaining competitive prices or tenders it is important that builders have exactly the same drawings, specifications schedules and documents. If they vary or are incomplete the prices you obtain will be meaningless. This is critical.
Only obtain prices from builders that you know or have researched thoroughly as being excellent in this type of work and who are not too far away. That way you will be confident that you will get a great result from whoever wins the tender.
Your Architects should check the builders who should have a demonstrable track record of carrying this out for other clients. Good tight pricing carried out by your Architects acting as project managers will save you a lot of money and you will end up with a tightly controlled project.
When all the prices are received they should be analysed carefully. Often small anomalies can be sorted out at this time which may include some repricing of some small items.
It is time to go back to your feasibility to make sure that the money is still on target. If not you will need to adjust the building price.
It should also be even-handed. It is not good to have a contract that is less fair on one party. You will end up by suffering the consequences! After all it is you who is taking the risk!