Formal:
* Bargain
* Negotiate
* Dicker
* Wrangle
* Chaffer
* Horse-trade
* Dispute (if the haggling is more about conflict than price)
Informal:
* Argue
* Squabble
* Bicker
* Nitpick
* Haggle over every penny
* Drive a hard bargain
* Get the best deal
* Cut a deal
The best synonym to use will depend on the context and the tone you are trying to convey.