Back in SR3, I remember you could haggle the price of a contract or anything with the skill negociation. It was detailed and everything.
Is there such thing in SR4 (detailed rules using negotiation)?

As a side note, I'd like to know why I can't edit my avatar.

Detailed? Hmm, no not really. There are details for applicable dice pool modifiers, but not exactly what it means to win the opposed test or how much you would be able to lower/raise price of something. Since it will often vary from negotation to negotiation, it's just as well.

As a rule of thumb though, each net hit in a negotiation could change the price by say 5% of total. Thus if the Johnson is normally willing to pay 10k the runner could get 11k with two net hits.
Keep in mind though that most Johnsons never start with their actual offer (what they think is a good price), they will usually lowball the price. If the johnson have 9 dice I'd probably let him start his first genuine offer at 8500 nuyen. At a break even negotation test the price would rise to 10k and the players would probably think they've made a good deal, just like their characters would think since the Johnson is a good negotiator.

Using the Denver modules as a reference, the 5% per hit is not a bad baseline to go with. Some of the modules actually say 10%, others give a hard nuyen value instead (usually 500 per hit at the highest); there's even listed limits in some, saying maximum 30% or even 50% depending on other factors, or just a hard nuyen amount. Heck, some mission modules even say that negotiation only ups what the Runners get as a retainer, and there's at least one where the fee is completely nonnegotiable.

In general, what Friend said is a good idea. Just keep in mind that you can tweak that even a little, or if you think the particular Mr. Johnson doesn't have the extra funds available then the runners can't negotiate at all on the fee. Not all Johnsons can haggle like a used car dealer.

But then, there's also other options too. Gear and/or services as well, though these shouldn't be too much more than what might have been earned in nuyen. Doing a job for a street doc could warrant a free patch job, for example.