Is the value of material components dependent on the price actually paid for the item? If I find a 50 gp diamond and decide to pay 300 gp for it (even though no NPC would ever buy it at that price), can I use it to cast Revivify? And if I find a 300 gp diamond and somehow bargain it down to 200 gp can I use it for Revivify? 

As long as the item's actual value fits the necessary requirements you can use it for a spell component. Example A = No, Example B = Yes.

Yeah, just to piggy back on the other reply, actual worth and sale price can and should be independent from one another. Bargaining for less or more is a common thing done when selling or buying items in dnd. Actual worth is determined by the DM, so just defer to them for what the diamond is actually worth. But yeah, totally cool if you roll a DC 30 persuasion check and haggle that 300gp diamond down to 50gp and charm the jeweler. 

I just hope the DM is generous about appraising your material components.

Would be potentially bad if you only figured out that nice old lady at the store swindled you and your "300gp diamond" is a fancy glass bead when you tried casting the spell. >.>