When I worked in academia, we were told to purchase all our Mac kit from a particular supplier, because they offered advantageous pricing to the Southern Universities Purchasing Consortium - basically a bunch of universities that got together and agreed that in exchange for exclusivity, supplier X would offer excellent prices. I used to get monthly bollockings from the purchasing department for ignoring the instruction and buying from my local AppleCentre - which gave better support and lower prices. "You must buy through SUPC", went the instruction, "because if you don't we'll lose the advantageous pricing". Yes, they really said that.
Just lately I've started buying stuff from a supplier I dealt with years ago. The reason's simple: I figured it was about time to challenge our usual supplier, so I simply chucked three specs at my former vendor and said: "Gimme a price". Of course, I didn't give him any clues as to what the target was. Three quotes duly arrived, all significantly lower than our normal vendor. I know he's not just a box-shifter, incidentally - he always used to give exemplary after-sales support too.
Now, for day-to-day stuff (i.e. things where consultancy and other value-adds aren't an issue) I don't flip-flop between suppliers. I simply tell them: "If you're the cheapest, you get the gig. If you're not, I'm not coming back to you for you to make a lower offer". This doesn't mean that I'll buy from anyone so long as the price is low - the point is that all of the suppliers I ask to quote meet all the other criteria (support, lead time, etc) and the remaining decision point is price.
So challenge your suppliers. You may be pleasantly surprised at the financial benefits.