I think that some clarification is going to be needed on the outset. Ethics in business are more situational than anything else. There are some basic ethical understandings, but for the most part, I think that the ethical considerations in business are dependent on legal ramifications as well as social concerns and also philosophical points of view. In terms of assessing the ethics of price fixing, I think that one has to examine the reasons behind why the prices are being fixed and who is participating in the fixing. Economic philosophies play heavily into this.
The ethical understandings of these economic philosophies might be understood differently than the moral philosophy that is traditionally understood when employing the term "ethics." For example, Classically Liberal economists view price fixing as inherently unethical because of its external intervention element. Other economic theories might embrace price fixing as a way to stabilize chaotic turbulence in marketplace prices.
I think that price fixing can be seen as unethical when it is designed to keep prices high for the benefit of the few at the cost of the consumers. I think that price fixing can also be seen as not being ethically sound when it is done in a collusion- like manner, without full public disclosure. In these cases, I think that one can see price fixing as unethical.