How much cheaper can you get shipping with negotiated rates and or a 3pl?

So a lot of small business owners use pirate ship. I was wondering how much lower negotiated rates can be in comparison to pirateship and what kind of volume would you need for the difference to be significant.

Alternatively when should one start considering a 3pl? How much volume do you need? How does the amount of variety of products you have factor in? How many dollars should you be netting per item before working with a 3pl?