HD and Lowes offer a price for "basic" install. Options tend to add up. Then usually the contractor has "extras" for repairs or site conditions that they will attempt to tack on. The installer gets paid very little, and if you have any problems, the store will make it the contractors responsibility. Keep in mind, most of the materials at the box stores are low grade but priced at mid grade.
Take hardwood flooring for example. Their bruce hardwood is a special "big box only grade", with a 5 year warranty vs the normal 20-25 year. There are so many defects that the box store gives you "10% more free" to make up for the 20% that needs to be culled. Their install starts at $3.50 per sq ft. Demo is extra (remove carpet, tile ect). Leveling or sanding the subfloor is extra. Cleats over staples is extra. Good underlayment is extra. Shoe molding is extra. Moving furniture is extra ($75 per piece per move). Delivery is extra. Get the picture?
The price you pay for install at the big box covers the cost of the heavily discounted materials, delivery, a sales person, the big boxes liability, the big box's millionaire tax bracket, the contractors liability, the contractor's labor, and 3-4 callbacks for complaints.
I can point you to some contractor's websites where are the drawbacks for the contractor are discussed too.
Anyhow, I'm MHIC licensed but kinda far from you. However my advice and estimates are free.