Good point, and 90+ percent of the time I won't kill for cheap. It took Maebius 20 gold and a morph to get me to go after all the TW's. When I got my contract I decided I'd charge around 4-5 gold per kill (I'll also accept creatures or something else I value around that amount), since it wasn't too much for people to get their hands on, but it also wasn't too cheap where I'd be getting contracts all the time. From PO being killed to the TW's being killed, how much time was between those two contracts? Probably well over a month.
Unless gold starts raining down from the sky I don't see much of an issue here.