The good thing about a rear derailleur is that it isn't under that much pressure, and if the crack went all the way through it would likely just fail to shift, or the tension spring go slack. PERSONALLY I would epoxy it and ride it for a little while longer until you find a new one. It may have just been installed WAY too tight, cracking that part trying to separate under the force of the bolt. Those gashes on the top are suspicious too, has it been wrecked? Rear derailleurs are cheap enough on ebay not to brake the bank.