No, there is no such thing as deteriorating drops.
From the dawn of time there were legends of subtle dependencies and rituals, that help a player boost his chances at games like Borderlands. When I was young, I had to convince my friend that shutting down Diablo 2 and restarting it does not boost his loot quality in Baal runs.
Of course, your theory makes more sense than dependencies like that one, but it is still wrong. It is just an unlucky streak and the way humans perceive randomness. Cognition is always working through expectation, and when you come up with a theory, your brain starts picking up evidence of it being true and ignoring conflicting facts.
The only way to check it for sure is to keep a list of every drop, color and price, and look at it when it becomes long enough - say, a hundred runs, but a thousand is better. Any sample less than a hundred would be random and would not demonstrate tendencies and chances.