Don't spend out of your range, that's about all there is. If she demands more than that you might want to think about who you're trying to marry.
I spent more money on my wife's rings because I wanted to get her a quality ring and diamond. We spent very little on my titanium ring, though, which offset part of the cost. Both of her rings are platinum, and each is more pricey than the diamond itself.
Diamond shoppingDoing it for size is absurd, to me almost all of the other factors are more important, clarity, colour, etc. Do you want to get a quality diamond or just a big diamond that looks like crap? If you're going to waste money on it at least waste it on the factors that make it look good.
Many distributors claim their diamonds do not come from slave trades or harsh labor, but who knows what's true. For the most part all you can go by is what they say and if you can't find anything refuting their claims then what can you do? All diamonds in the US are legally required to come through certified ethical supply chains, so if nothing else you can probably believe many sellers truthfully believe their diamonds have not come from unethical means. You can really pony up and get a Canadian diamond if you really want to be safe.
The best way to save money is staying out of brick and mortar stores. They're notorious for both overpricing and overrating, a double whammy on your pocketbook. Here's lots of good and common points on how many diamond sellers may try to bump their prices:
https://www.diamondhelpers.com/ask/0040-scams.shtml Shopping online from places that offer independently rated diamonds is easy enough, and oddly more trustworthy. Do some research on who is best to buy from, you'll come up with a few sites that have great reputations. After purchasing take it to get appraised, or make sure before you buy you can get it appraised to make sure things match up.
Remember, you can always buy the diamond and then have it set on a ring anywhere.