Hiring a contractor based on a 'grand total' bid or estimate may cost you.
Assuming that most customers can not distinguish between the costs of labor and materials, less-than-honest contractors are known to mark up materials or exaggerate material costs. In theory, this is crazy because they price themselves out of a job.
When shopping bids for 'large' projects, attempt to obtain from each contractor separate estimates for labor and materials. Materials and other non-labor expenses are constant regardless of the contractor performing the work. Either pay for the majority of materials yourself and/or let the contractor know that you will require copies of receipts during or after the project in order to reconcile total materials costs with what was actually purchased or used to complete the work. For example, receipts for a kitchen remodel project would not normally include roof shingles and tar paper.
The customer can accomplish retail material purchases from the supplier with a cash card, store gift card, over the phone with a credit card, or in person at point of purchase.