First of all coinbase is not going to be the best way to buy bitcoin if you were dealing with high amounts. If you were dealing with high amounts, go with GDAX. Essentially same thing, but with no fees.

I have never heard anyone negotiate rates with coinbase before. I'm pretty sure that they are a market maker, therefore it would be hard for them to make their rates flexible for negotiation and everyone will just start doing it.

But it's worth a try, if you have a large amount. Doesn't hurt but the response will most likely but negative.