it really is a loaded question this one, and in times of austerity its hard to argue against the economic advantage...
personally, where possible i rotate all tyres for even wear and replace all 4 with new, known brand tyres. but if i was feeling the pinch a little, i cant say hand on heart I wouldnt chose the cheaper option to keep me on the road. If its something your going to do, then at least give the tyre a good visual check over before its fitted, and if in any doubt, reject it before its fitted and balanced.
saying that, on my old land rover with offroad tyres, I will always fit a retread tyre, as im much more likely to damage and rip the sidewall out. and they are half the price to replace. never had one fail, or suffered tread seperation, whih is where the main tread of the tyres becomes unstuck from the carcass and peels off like an orange.