If you are spending any significant amount on building materials, you really should take the time to get a few quotes. First, neatly generate a printable list that's very clear, and broken down into specific products and materials.
for example
15 bags- R-21 Kraft faced fiberglass batts 15 X93" 10 pieces per bag
20 bags- r-30 " " " " 15x48" 12 " " "
1- box Arrow T-50 ( or equal) 3/8" 5000 count staples
48- Foam baffle tray vents 22-1/4" x 48"
1- case (dozen) standard Great Stuff (or equal) expanding foam
Include your contact information, your address, and that you want a price that includes delivery. Now hand deliver it to the big box stores and at least two local lumberyards. At the yards, as to speak to whomever handles contractor sales, and take a minute to introduce yourself. Let the salesperson know that you have dropped the list off at a few local suppliers, that you a serious buyer who will be needing more stuff in the future AND will pay cash.
Don't be fooled by the lure of the big box store. In my area, they can be competitive in some categories, but when it comes to things like kitchen cabinets, doors and trim, or having 1/5th of the lumber that a real yard stocks, they aren't even close.