In the United States, most construction bids are for a stipulated sum (also known as "lump-sum" or "fixed-fee" bids). This amount includes all materials, labor, and other costs.
When bidding these jobs, you need to carefully go through the plans to determine what quantity of each type of device is required, how much cable is required to connect them, and what type of head-end equipment is required. You also need to carefully review the specifications to determine what types of products that you must provide, how they must be installed, and what types of support items (training, spare parts, etc.) that you must provide. You then need to arrive at a lump-sum amount that includes everything, including your overhead and profit.
On larger construction projects, it is most common for low-voltage integrators to work as a sub to the electrical contractor (EC) who in turn works for the general contractor (GC). In this case, you would submit your bid to the ECs that were bidding the project. In some cases, the GC will accept bids directly from integrators. The best thing to do is to place a call to the GC asking how that they would like bids for low voltage work to be submitted.