Will my suppliers reduce prices? Yes, especially on larger orders, repeated similar orders, and things they accidentally over-ordered to begin with. I also expect (and sometimes ask for) zero reduction for the small jobs, or things that I know are going to come with a lot of research on their part.
I negotiate with my insurance broker, my banker, both my phone companies, and the auto repair-shops. Sometimes I get a discount, sometimes I don't. If I take my car to the dealer for an oil change, I'm gonna pay retail. If I need oil change, tune up, transmission flush, gaskets, and a new alternator, I'm going to ask for, and usually get, a discount on the labor. That's because the dealer and I both know they have fixed pricing for those items based on a nation-wide average of the hours it takes to do each job. When they have the same tech, working in the same area of the truck, it takes a lot less time then it is priced at.
If I think a price is unreasonable for the market/labor/parts/quality combined, then I have no problem saying so and asking for a contractor to verify the numbers. Then I make a decision from there.
As an example, many contractors around here like to flat rate recessed down lights. I on the other hand would expect the reasonable cost of 6 cans installed in 1 upstairs room (with attic space), 1 switch, & 1 home run to be different than 2 cans each w/ 1 switch installed in 3 downstairs rooms (no crawl space), & 1 home run. If they come back as the same price on 2 line items of the same estimate, I'm going to ask them to recheck the numbers, even if I think they have underbid. I want to keep the contractor in business, I am not interested in keeping them in Armani. Is that unreasonable of me as a client?