Interesting question. I didn't get an engagement ring but that is because I was adamant that I didn't want one (I break things, really, I do) and I knew that once I was married, it would rarely see the light of day. However, my wedding band is worth £2000, but, DH only paid £370 for it because it has my grandmother's diamonds and gold, but was redesigned to fit my style.
But, basically, I think it is a variable thing. It isn't the price that counts, really, it is the sentiment. It is also something that should fit the style of the wearer. If the woman is ga ga over diamonds, then diamonds it should be, thus making the price higher. If the woman adores sapphires, then that is what it should be and the price would be less.
In the end, it should cost what the man can afford, in my opinion. Some say it should be 2 months salary, but, I think so much more goes into the whole decision than just cost.
Just my two cents/pence....