For me each deal is dealt with on it's own basis. If I really want the pieces the other side is offering, I can offer things that are worth a bit more, that I simply don't need. As G_F said, I just go to coolstuffinc.com etc. to get a good idea for the value, and each side figures out what they are willing to do in respect to that.
But, in the end, it's up to each side what and if they want to give up for what the other side is offering
. Sometimes, I've made proposals (and vice-versa) and they were immediately agreed upon. Other times, some negotiating was required.