Contractors build material cost from supplier quotes. A quote lists items, quantities, unit prices, freight terms, lead time and how long the price holds.
An electrical sub sends the fixture schedule to a lighting supplier. A mechanical sub sends the equipment schedules to several vendors.
Read each quote against the spec. Check that the supplier priced the specified products and all accessories. Check tax, freight and the expiry date.
Note what the supplier excludes. Startup, training, anchors and spare parts are often left out and still required by the spec.