I don't believe half the stuff I read about the large bounties placed on the heads of rats and such. The $500,000 price on Pistone has pretty much been dismissed as BS. On the flip side,It would be a smart move to put word out on the street that there was a huge reward for a snitch's death.
A mob Boss runs very little risk by putting this kind of stuff out there for several reasons:

1) No made guy is going to take the chance on doing it himself,since no Boss is going to risk a major RICO charge by even discussing it,let alone passing the order down.

2)Every rum-dum wannabe on the street figures that he will make major points,and get on the fast track to being made by doing the deed. Now you have a bunch of goofballs looking for the rat,and none can be linked to the Boss or Family members.

3) If by some quirk,a street punk is succesful,what is he going to do? He can't collect,because to do so would involve some Family member conceding that the reward existed in the first place,which again, is a RICO charge. The other scenario is a meeting at some deserted location at which his reward will be inserted behind his ear.
Even this is unlikely,because how can anyone be certain that it isn't an FBI sting?