I am most definitly not an expert, but one thing I am pretty sure of is that the improvement vs price graph is most definitly going to be a sigmoid curve. I know a speaker cable costing 10p a metre will be crap compared to one costing £5/m, but £5m vs £10/m? Or £50/m vs £100/m? Whatever product you buy, the cheapo ones will be bad, and there will be a huge improvement above a certain price, then its the law of diminishing returns. Ultimately there will be a few bargains, or dogs in there, but I think its a good rule to use. You just need to know where the improvements really start to come in.