Hm, in principle I'm actually sympathetic to the idea that the wishes of the victim or his/her relatives should have some bearing on the degree of punishment meted out to the perpetrator. This has been common in lots of cultures (like 'weregild' in Anglo-Saxon or Viking law, whereby a murderer could buy his own life by offering suitable compensation to the bereaved family - although he may well still have been banished or served some severe corporal punishment as well, I'm not sure). The big problem with this, as you say, is that very rich defendants can basically buy a pardon from a poor family. You could also have people acting on behalf of the defendant making threats to get them to drop it (although I guess that can happen anyway in cases where the complainant has the power to drop charges).
I dunno, it's a tricky one. I think there is some case to be made for the wishes of the victim or their representatives to be taken into account in sentencing. This wouldn't necessarily result in aggrieved mothers pleading for the death penalty; in fact you quite often get people saying they forgive the defendant, believe he's genuinely remorseful and can't see any purpose in his being incarcerated. Just a thought.